In the very beginning of his writing, Luke lays out his purpose in recording the life and ministry of Jesus. Though he addresses Theophilus, the style indicates he expected many people to read his book, so that all believers would have certainty about the things they have been taught. This certainty comes from having “followed all things closely” (v. 3) that came from “those who from the beginning were eye-witnesses and ministers of the word” (v. 2). So, while Luke is giving us a second-hand account, his sources are primary sources who really saw Jesus do amazing things.
You speak only the truth, O Lord. You give us certainty in an uncertain world.