Luke 1:4

Mightest know. Gr. epiginosko, “to know fully.” That is, Theophilus was to have knowledge in addition to what he already knew of “those things” wherein he had “been instructed.”

Certainty. Gr. asphaleia, that which will not fall, from the two words sphallo, “to totter,” “to fall,” and the prefix a, “not.” There is “certainty” to the facts of the Christian faith, and he who believes in them will be steadfast and secure against error.

Been instructed. Gr. katecheo, “to instruct,” or “to teach orally”; literally, “to sound down upon.” Katecheo is the source of our word “catechize.” It is rendered “informed” in Acts 21:21, 24, “instructed” in Acts 18:25, and “taught” in Gal. 6:6. This word may imply that Theophilus had thus far received only oral instruction, such as might appropriately precede baptism. It is possible that he was one of Luke’s converts, one whom Luke had “catechized.” Or it may be that Luke wrote these things out to meet false reports against Christianity.