In this first Chapter, Luke spells out very clearly why he has written this book
1Inasmuch as many have undertaken to compile a narrative of the things that have been accomplished among us, 2just as those who from the beginning were eyewitnesses and ministers of the word have delivered them to us, 3it seemed good to me also, having followed all things closely for some time past, to write an orderly account for you, most excellent Theophilus, 4that you may have certainty concerning the things you have been taught.Why does Luke write?
So that Theophilus (to whom he is writing) may have certainty concerning the things [he has] been taught. Lots of things had been written down about Christ and his ministry, and Luke, looking them has discovered/decided that having an orderly account of all this would be in order. This would help his readers to be certain of the things they know about God and Jesus' ministry here on earth.
He knows that since the beginning, eyewitness have delivered truth to us. That ministers of his (God's) word have delivered God's word to us.
From BW Johnson's commentary we learn this
1. Forasmuch as many. Verses 1-4 are an introduction. They explain that already many narratives of Christ had been written, that these were by eye witnesses and ministers of the word, that Luke had made a careful examination of all these sources of information, and thought it good, "having traced all things accurately from the first, to write them out in order." We thus learn that at least as early as twenty-seven years after the death of Christ (see Introduction to Luke) many histories of eye witnesses and ministers had already written, of which only two, Matthew and Mark, have come down to us.Another commentary you might want to consider reading is that of Gills. He is quite indepth. What Luke wants for Theophilus, according to Gills, is this :
3. Most excellent Theophilus. The name means "A lover of God." He is named in Acts 1:1, but of him nothing more is known
The end the evangelist had in writing this Gospel, and sending it to Theophilus, was, that he might be more strongly assured of and more firmly established in the truths of the Gospel.Isn't that the end that all of us want when reading the bible? To understand it more fully? To know that it is truth?
We need, as believers, to be strongly assured of God's word. We need to be firmly established in the truths of the Gospel. It is the Gospel by which we stand!