Theophylact of Ohrid
Catena Aurea · Lukasevangelium 1
Forasmuch as many have taken in hand to set forth in order a declaration of those things which are most surely believed among us, Even as they delivered them unto us, which from the beginning were eyewitnesses, and ministers of the word: It seemed good to me also, having had perfect understanding of all things from the very first, to write unto thee in order, most excellent Theophilus, That thou mightest know the certainty of those things, wherein thou hast been instructed.
(Præf. in Luc.) By these words it is plainly implied, that Luke was not a disciple from the beginning, but became one in course of time; others were disciples from the beginning, as Peter, and the sons of Zebedee.