Luke 1:4

Wherein thou hast been instructed - Κατηχηθης - In which thou hast been catechized. It appears that Theophilus had already received the first elements of the Christian doctrine, but had not as yet been completely grounded in them. That he might know the certainty of the things in which he had been thus catechized, by having all the facts and their proofs brought before him in order, the evangelist sent him this faithful and Divinely inspired narrative. Those who content themselves with that knowledge of the doctrines of Christ which they receive from catechisms and schoolmasters, however important these elementary instructions may be, are never likely to arrive at such a knowledge of the truth as will make them wise unto salvation, or fortify them against the attacks of infidelity and irreligion. Every man should labor to acquire the most correct knowledge, and indubitable certainty, of those doctrines on which he stakes his eternal salvation. Some suppose that St. Luke refers here to the imperfect instruction which Theophilus had received from the defective Gospels to which he refers in Luke 1:1.

The certainty - Have full evidence or proof of.

Been instructed - By the preachers of the gospel. The original word is the one from which is derived our word "catechism - been catechized;" but it does not denote here the "manner" in which the instruction was imparted, as it does with us, but simply the fact that he had been taught those things.