John Chrysostom

Or it may be, "That thou mightest feel certain and satisfied as to the truth of those things which thou hast heard, now that thou beholdest the same in writing."

Catena Aurea by Aquinas ↗

Theophylact of Ohrid

I understand this in two ways. First, thus: previously I instructed you, Theophilus, without writing, and now, transmitting the Gospel to you in writing, I confirm your mind so that it would not forget what was delivered without writing. Second, thus: we people often have the custom, when someone tells us something without writing, of suspecting him that perhaps he is even speaking falsehood; but when he writes down his words, we believe that he would not have written them if he were not boldly confident in the truth of his words. …

Commentary on Luke ↗

Bede

Not, however, of any new or unknown things does he promise to unfold the account to this same Theophilus, but to express the truth of the words in which he was instructed. Of course, so he might recognize the order in which whatever was done or said by the Lord or about the Lord. For he who wishes to be perfect ought not only to believe in Christ but also to understand the order of his eternal divinity and his temporary dispensation.

On the Gospel of Luke ↗

Bonaventure

Fourth, as to the consummative element, which was the understanding of truth, he adds: That you may know the truth of those words in which you have been instructed, know, namely, more fully. And this was that in which the intention of the Evangelist in writing came to rest: whence 2 Timothy 3: "All Scripture divinely inspired is useful for teaching, for arguing, for correcting, for instructing in justice, that the man of God may be perfect, equipped for every good work"; and this end Luke sets forth with respect to all whom he …

Commentary on Luke, Chapter 1 ↗