Even though our English word for Church came from old English words for places of worship, Christianity did not always have buildings. This table lists early church buildings which were built or adapted for the purpose of holding Christian worship. Prior to 311 Christianity was often illegal and persecuted so the assemblies were most often held in the houses of the members. Most of the earliest buildings in the table are in ruins today, the exception is the Cathedral of Saint Domnius,which was built as a Mausoleum for Diocletian and ironically is now the oldest Catholic Cathedral in use.

Chronicling old buildings does not tell us much about the Church, it does show that as the Church became legal and even recognized it became more of an institution. We also see that it displaces pagan religions and often took over those places oww worship.

 Oldest PartBuildingLocationCountryContinantDenominationNotes
Third Century     
  c. 241  Dura-Europos church Dura-Europos Syria Asia Early Church House church. Several walls still standing, oldest images of Jesus discovered within the surviving frescoes of the large baptistry room. Fragments of parchment scrolls with Hebrew texts unearthed containing Christian Eucharistic prayers closely connected with the prayers in the Didache.
  295–305 Cathedral of Saint Domnius Split Croatia Europe Roman Catholic Built 295–305 as the Mausoleum of emperor Diocletian, is the second oldest structure used by any Christian Cathedral. It is one of the best-preserved ancient Roman buildings standing today. It was built as a mausoleum for Diocletian, the last famous persecutor of the Christians, who was interred here in 311 AD. The Christians got the last laugh, destroying the emperor's sarcophagus and converting his tomb into a church. Consecrated as a Church in 641 AD, it is regarded as the oldest Catholic cathedral in the world that remains in use in its original structure, without near-complete renovation at a later date.
  late 3rd century Megiddo church Legio near Tel Megiddo Israel Asia Early Church Ruins discovered on prison grounds in 2005, possibly oldest building constructed as a church ever discovered. It retains mosaic floor decoration with Christian motifs – possibly the oldest building with this feature.
  late 3rd-eartly 4th century Aqaba Church Aqaba Jordan Asia Early Church Ruins uncovered in 1988; Remains of an early basilica. Considered to be the world's first purpose-built church.
Fourth Century     
  301 (tradition); current church: late 5th century Etchmiadzin Cathedral Vagharshapat Armenia Asia Armenian Apostolic Church According to scholars it was the first cathedral (but not the first church) built in ancient Armenia. It is also the oldest Cathedral in the world built for that purpose.  Although the current sanctuary was erected in the 1600's, remnants of the 4th-century altar have been unearthed beneath the present structure. In 301 Armenia became the first nation in the world to adopt Christianity as its official religion.
  306 Rotunda of St. George Thessaloniki Greece Europe Greek Orthodox Currently serves as a museum, although the Greek Orthodox Church has access to the church for various festivities. The structure was built in 305 and consecrated later in the century.
  c. 309 Santi Cosma e Damiano Rome Italy Europe Roman Catholic

Santi Cosma e Damiano was the first church to be founded in the Forum. In 527, Pope Felix IV converted a rectangular apsed hall in the Forum of Peace into a church by the simple addition of an apse mosaic (which survives) and some church furnishings.

Although Christianity had been the official religion of the empire since the late 4th century, the placement of a church in the Imperial Forum marked a turning point: the power and wealth of the Church had finally reached a point that it was able to take over what had been the center of pagan worship in ancient Rome. http://www.sacred-destinations.com/italy/rome-santi-cosma-e-damiano

Occupies the former space of the Temple of Romulus in the Roman Forum.
  313 Saint Sofia Church Sofia Bulgaria Europe Bulgarian Orthodox The church was built near the Amphitheatre of Serdica. 343 in the building took place the Council of Serdica that was attended by 316 bishops.
  326 Panagia Ekatontapiliani Parikia Greece Europe Greek Orthodox Panagia Ekatontapyliani (also known as the Church of 100 Doors) is a historic Byzantine church complex.
  326 Sts. Constantine & Helena Cathedral Yabroud Syria Asia Syriac Catholic Originally built as a pagan temple dedicated to Baalshamin during the first millennium BC, it was converted to a church when St. Helen passed through the city on her way back to Rome from Jerusalem. The church was damaged during the Syrian Civil War.
  327–329 Stavrovouni Monastery Larnaca Cyprus Europe Greek Orthodox It was founded by Saint Helena the mother of the Byzantine Emperor Constantine the Great at the place where after a miracle she found the lost Holy Cross on her way back from her pilgrimage to the Holy Land. Saint Helena went on the pilgrimage after the First Ecumenical Synod in Nicaea (325). The monastery has a piece of the Holy Cross left there by Saint Helena.
  333 St. Peter's Basilica Vatican City Vatican City State Europe Roman Catholic Largest church in the world and the seat of Roman Catholicism, the current church (1635) is built on the site of the original basilica and the grave of St. Peter (who was martyred in Rome). The actual grave is visible in excavations under the church. The original floor and columns are visible in a space under the 17th-century floor.
  335 Church of the Holy Sepulchre Jerusalem   Asia Simultaneum Originally completed in 335, the building suffered extensive damage and was almost completely destroyed in 1009. The church was rebuilt in 1038.
  339 Church of the Nativity Bethlehem Palestine Asia Simultaneum It was commissioned in 327 by Constantine and his mother Helena over the site that was traditionally considered to be the cave that marks the birthplace of Jesus. The original basilica was completed in 339.
  340 Santa Maria in Trastevere Rome Italy Europe Roman Catholic The first sanctuary was built in 221 and 227 by Pope Callixtus I and later completed by Julius I, but the oldest parts of the present church, the wall structure and floors, date from 340 AD.
  340 Cathedral of Trier Trier Germany Europe Roman Catholic Oldest cathedral in Germany, and still in use today. Its construction was overseen by Bishop Maximin of Trier, the construction of the grandest ensemble of ecclesiastical structures in the West outside Rome. A few 4th-century elements remain in a mainly Romanesque building.
  356 Monastery of Saint Anthony Eastern Desert Egypt Africa Coptic Orthodox Partially destroyed in the 11th century and rebuilt; very little of the original structure remains.
  363 Mar Mattai monastery Mount Alfaf Iraq Asia Syriac Orthodox Extensively rebuilt over the years with the earliest portions built during the 4th century.
  364 Basilica di San Lorenzo Milan Italy Europe Roman Catholic When built, it was the biggest circular church building standing. It was an inspiration for the Hagia Sofia in Constantinople. Some ancient features remain.
  c. 374 Basilica di San Simpliciano Milan Italy Europe Roman Catholic Exterior walls are original from the late 4th century. Roman basilica windows as in Cathedral of Trier are still visible on the flanks of the basilica.
  397 Mor Gabriel Monastery Midyat Turkey Asia Syriac Orthodox Built on the ruins of an old Assyrian temple, the property is currently subject to a dispute between the Turkish government and the church.
  380 Basilica of Saint-Pierre-aux-Nonnains Metz France Europe Roman Catholic Originally built to be part of a Roman spa complex, the structure was converted into use as a church in the 7th century when it became the chapel of a Benedictine monastery. It is the oldest church building in France.
  c. 382 San Nazaro in Brolo Milan Italy Europe Roman Catholic One of the earliest Latin cross buildings in western Europe; retains few ancient features.
  386 San Paolo fuori le mura Rome Italy Europe Roman Catholic One of the four major Constantinian basilicas in Rome, and the only one to retain its antique character, the basilica was heavily damaged by an 1823 fire. It was rebuilt true to form, however, so it remains one of the best "preserved" Constantinian basilicas in the Roman world. All that remain of the ancient basilica are the interior portions of the apse and the triumphal arch.
  398 Santi Giovanni e Paolo Rome Italy Europe Roman Catholic A 4th-century basilica was erected on the site of one of the most well-preserved "house churches" of early Christianity (dating from the 1st, 2nd or 3rd century), with intact fresco decoration. The houses are part of a museum currently, but the basilica still functions as a church.
  c. 4th century Basilica di Sant'Eustorgio ruins Milan Italy Europe Roman Catholic Some ruins remain of the apse of the ancient basilica.
  4th century Church of St. George Sofia Bulgaria Europe Bulgarian Orthodox Built by the Romans with many later additions, St. George is the oldest building in Sofia and the only one dating from the Roman era. Still a functioning church.
  4th century Aula Palatina (Konstantinbasilika) Trier Germany Europe Evangelical Church in the Rhineland A Roman palace basilica that was built by the Emperor Constantine at the beginning of the 4th century. The basilica contains the largest extant hall from antiquity. The church was converted to Protestant use from its original Roman Catholic use in 1856.
  4th century Santi Nereo e Achilleo Rome Italy Europe Roman Catholic Adjoins the Baths of Caracalla at the site where St. Peter is said to have dropped the cloth covering his wounds upon his removal from the Mamertine Prison. First referenced in 336, a 15th-century basilica sits on its original site; only the columns are original.
  4th century Santa Pudenziana Rome Italy Europe Roman Catholic Retains original and unique late Roman mosaic decoration.
  4th century San Clemente al Laterano Rome Italy Europe Roman Catholic The present basilica was built just before the year 1100, but beneath it is an intact 4th-century basilica that had been converted out of the home of a Roman nobleman, part of which had in the 1st century briefly served as an early church. This ancient basilica retains fresco decoration. The basement the house had in the 2nd century briefly served as a mithraeum. The home of the Roman nobleman had been built on the foundations of a republican era building that had been destroyed in the Great Fire of Rome in 64 AD.
  4th century Santa Costanza Rome Italy Europe Roman Catholic Built under Constantine as a mausoleum for his daughter, Costanza. Retains many original mosaic decorations, and her porphyry sarcophagus is a gem of the Vatican Museum.
  4th century Santi Quattro Coronati Rome Italy Europe Roman Catholic Sanctuary is located in an isolated green area of Rome, so it retains the ambience of the area in antiquity.
  4th Century Church of Our Lady Mary of Zion Axum Ethiopia Africa Ethiopian Orthodox Originally built in the 4th century, it has been rebuilt several times, most recently in the 17th century during the reign of Emperor Fasilides.
  4th century Mar Sarkis Monastery Ma'loula Syria Asia Syriac Catholic Built in the 4th century on the remains of a pagan temple, it likely predates the Council of Nicea (325 AD) as evidenced by the fact that it has a round altar, which was prohibited at the Council. The monastery suffered extensive damage and desecration in the Syrian Civil War.
  4th or 5th century Kasagh Basilica Aparan Armenia Asia Armenian Apostolic Original building retains many ancient features.
  4th century Chora Church Istanbul Turkey Asia Eastern Orthodox Very little remains of the 4th-century structure, but it contains impressive 13th-century mosaic decoration. It was converted to a mosque in 1501 and has been a museum since the time of Ataturk.
  4th century Hagia Irene Istanbul Turkey Asia Eastern Orthodox Now a museum and concert hall (due to its excellent acoustics). Its excellent state of preservation is due to the fact that it lies inside the Ottoman palace complex (Topkapı Palace).
  4th century Jubail Church Al-Jubail Saudi Arabia Asia Assyrian Church The government hides it from locals and bans foreigners from openly visiting it – even archaeologists.
  4th or 5th century Church of St Peter Antioch (Antakya) Turkey Asia Syriac Orthodox Cave church used by very first Christians of Antioch, where St. Peter presided as bishop. Known by its original Aramaic name of Knisset Mar Semaan Kefa. Extensively reconstructed, with oldest surviving elements from 4th or 5th century.
  433 Valkum (Fenekpuszta)          

Valkum (Fenekpuszta) Built before 433 AD remains of a Bazilika in Hungary near lake Balaton

https://en.wikipedia.org/wiki/List_of_oldest_church_buildings 12/29/15