38 For [it is noticeable that] again in Cana of Galilee, after He had turned the water into wine, this same John tells us how His disciples believed on Him. The narrative of that miracle proceeds thus: “And the third day there was a marriage in Cana of Galilee; and the mother of Jesus was there. And both Jesus was called and His disciples to the marriage.” Now, surely, if it was on this occasion that they believed on Him, as the evangelist tells us a little further on, they were not yet His disciples at the time when they were called to the marriage. This, however, is a mode of speech of the same kind with what is intended when we say that the Apostle Paul was born in Tarsus of Cilicia; for certainly he was not an apostle at that period. In like manner are we told here that the disciples of Christ were invited to the marriage, by which we are to understand, not that they were already disciples, but only that they were to be His disciples. For, at the time when this narrative was prepared and committed to writing, they were the disciples of Christ in fact; and that is the reason why the evangelist, as the historian of past times, has thus spoken of them.
Source: The Harmony of the Gospels (New Advent)