Many Christians quote Matthew 28:19 to show that Jesus taught that God is a Trinity, but this verse cannot be found in any manuscript of the Gospel of Matthew until the fourth century, proving it was later inserted to support the Trinity doctrine.

The question presumes that there are complete copies of Matthew that omit Matthew 28:19 before the fourth century, which is not supported by evidence. In fact, there are no extant manuscripts of Matthew that contain all of its chapters prior to the fourth century. The earliest manuscripts available do not include chapter 28, but this does not imply that the original Gospel lacked this verse. Furthermore, the writings of early Church Fathers indicate that the Trinitarian baptismal formula was known and used, suggesting that it was indeed part of the original text of Matthew. This historical usage supports the claim that Matthew 28:19 was not a later insertion but rather an authentic part of the Gospel.