When St. Luke wrote his Gospel, the recipient he had in mind was Theophilus. It’s an odd name—odd, in that it’s very rare name, not often, if ever, found in any historical records—outside of Luke’s writings, there is hardly anyone named Theophilus, especially anyone not connected to the Church. The name means “friend of God” or “one who loves God,” and for that reason, some surmise that Luke’s recipient is not a specific person, the epithet “most excellent” aside (as perhaps referring to a government official), but any person who is a friend of God or who loves God; as they might explain it, Luke’s Gospel and Acts are written for the believer—he wrote them for you, dear hearer.
To what end did Luke write to Theophilus? “[T]hat you may know the certainty of those things in which you were [catechized].” (Luke 1:4) Theophilus was someone who had been catechized—prepared for Holy Baptism in the ways of the ancient church—and Luke was writing to the catechized friend of God so that he would know for certain the reliability of the words by which he had been catechized. It’s a stated purpose in several places in the Scriptures that the Word of God is given in order that faith would be created in an individual. (cf. John 20:31) And in fact, God’s desire is that all men should be saved and come to a knowledge of the truth. (cf. 1 Timothy 2:4) In essence, these are all the same thing: God wants all men saved, to that end, He sent His Son and gave His Word in order to redeem mankind and give them faith.