Where did the first church begin?
Jesus gathered and trained his disciples during his earthly ministry, and he spoke of building his church on the confession that he is the Christ (Matthew 16:18). After his resurrection and ascension, his followers waited in Jerusalem as he had told them.
At Pentecost, the Holy Spirit came upon the gathered disciples. Peter then proclaimed Jesus' death and resurrection, and thousands responded in faith and baptism. Those believers devoted themselves to the apostles' teaching, shared life, prayed, and broke bread together. This Jerusalem community is normally understood as the first visible Christian church (Acts 2:1-4; Acts 2:41-42).
And they continued stedfastly in the apostles' doctrine and fellowship, and in breaking of bread, and in prayers.
It was not one modern denomination
The first church was not identical to any present-day denomination. It was a growing Jewish community centred on Jesus in Jerusalem, before the gospel spread widely among Gentiles. As believers carried the message beyond Judea, churches formed in places such as Antioch. It was in Antioch that Jesus' followers were first called Christians (Acts 11:26).
That doesn't mean history is unimportant. Many churches rightly value their links with early Christian worship, teaching, and leadership. Still, a claim to ancient roots by itself cannot settle whether a church is faithful. Every community, old or new, needs to be measured by the gospel handed down through the apostles.
What makes a church true?
The New Testament points first to Jesus, not to a later organisation. Paul says that no foundation can replace Christ, and he describes God's household as built on the apostles and prophets, with Christ as its chief cornerstone (1 Corinthians 3:11; Ephesians 2:19-20). A true church keeps returning to who Jesus is and what he has done.
That includes holding to the apostolic message rather than reshaping it to suit the times. Paul warned the Galatian Christians against accepting a different gospel, even if it came with impressive claims (Galatians 1:8-9). Christians may disagree sincerely about baptism, church government, communion, and worship style. Those questions matter, but they are not the same as abandoning the gospel of Christ.
For other foundation can no man lay than that is laid, which is Jesus Christ.
Look for Christlike fruit
Jesus said that love among his disciples would show the world that they belonged to him (John 13:34-35). So it is fair to ask practical questions. Does this church teach Scripture honestly? Does it point people to Jesus rather than to human status? Does it make room for repentance, grace, truth, prayer, and care for others?
The church is also bigger than any one local congregation. Paul pictures believers from different backgrounds as one body, brought into that body by the Spirit (1 Corinthians 12:12-13). You can appreciate a church's history without treating its age as a guarantee. Read the New Testament for yourself, pray for wisdom, and seek a community where Christ is trusted and followed.
By this shall all men know that ye are my disciples, if ye have love one to another.