Matthew, Mark, Luke, and John wrote their accounts. Luke completed his writing with the Acts of the Apostles. In the early days, a number of different accounts about Jesus' life existed. By the second century AD, religious leaders confirmed the four Gospels we know today as the only genuine, authoritative, and inspired accounts for believers. We know much more about how the New Testament developed than we do about the Old Testament.
Many ancient manuscripts exist that provide clear indications of what likely occurred.