13 Or “know the truth about”; or “know the certainty of.” The issue of the context is psychological confidence; Luke’s work is trying to encourage Theophilus. So in English this is better translated as “know for certain” than “know certainty” or “know the truth,” which sounds too cognitive. “Certain” assumes the truth of the report. On this term, see Acts 2:36; 21:34; 22:30; and 25:26. The meaning “have assurance concerning” is also possible here.
14 Or “you heard about.” This term can refer merely to a report of information (Acts 21:24) or to instruction (Acts 18:25). The scope of Luke’s Gospel as a whole, which calls for perseverance in the faith and which assumes much knowledge of the OT, suggests Theophilus had received some instruction and was probably a believer.

In other versions

that you may have certainty concerning the things you have been taught.

so that you may know the certainty of the things you have been taught.

so that you may know the certainty of the things about which you have been instructed.

so you can be certain of the truth of everything you were taught.

so that you may know the certainty concerning the things about which you were taught.

That thou mightest know the certainty of those things, wherein thou hast been instructed.

Public Domain