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.

Or it would seem that the two disciples who followed Jesus were Andrew and Philip.

Let us see what follows: "Behold the Lamb of God." This John said, and the two disciples heard him speak, and followed Jesus. Then Jesus turned and saw them following, and saith unto them, "What seek ye?" And they said, "Rabbi (that is to say, being interpreted, Master), where dwellest Thou?" They did not follow Him in such manner as that they should cleave to Him; for it is plain when they cleave unto Him, for He called them from the ship. For one of the two was Andrew, as you have just heard, and Andrew was the brother of Peter; and we know from the Gospel that the Lord called Peter and Andrew from the ship, saying, "Come ye after me, and I will make you fishers of men." And from that time they clave unto Him, so as not to go away. On the present occasion these two followed Him, not as those who were not again to leave Him, but to see where He dwelt, and to fulfill the Scripture: "Let thy foot wear out the threshold of His doors; arise to come to Him continually, and be instructed in His precepts." He showed them where He dwelt: they came and remained with Him. What a blessed day they spent, what a blessed night! Who can make known to us those things which they heard from the Lord? Let us also build in our heart, and make a house into which He may come and teach us, and have converse with us.