John 1:40
“One of the two who heard, and followed Him, was Andrew, Simon Peter's brother.”
Wherefore then has he not made known the name of the other also? Some say, because it was the writer himself that followed; others, not so, but that he was not one of the distinguished disciples; it behooved not therefore to say more than was necessary. For what would it have advantaged us to learn his name, when the writer does not mention the names even of the seventy-two? St. Paul also did the same. “We have sent,” says he, “with him the brother,” (who has often in many things been forward,) “whose praise is in the Gospel.” Moreover, he mentions Andrew for another reason. What is this? It is, that when you are informed that Simon having in company with him heard, “Follow Me, and I will make you fishers of men”, was not perplexed at so strange a promise, you may learn that his brother had already laid down within him the beginnings of the faith.
Source: Homilies on the Gospel of John (New Advent)