Luke wrote this Gospel so that you [Theophilus] may have certainty concerning the things you have been taught [i.e., catechesis] (1:4). It proceeds with a barrage of questions.
The first comes from Zechariah: How shall I know? (1:18). This is a question of knowledge. The angel Gabriel answers that we shall know by the word of God (1:19).
The second comes from Mary: How will this be? (1:34). This is a question of agency, of how the Son of God can become human. Gabriel answers that it will be by the power of God (1:35).
The third comes from Elizabeth: Why me? (1:43). Why should the gospel of Jesus Christ come to her? Mary answers that it has to do with God's choice, election, mercy, and salvation (1:47–55).
The fourth has to do with John: What then will this child be? (1:66). Zechariah answers that he will be a prophet (1:76–79). John is not the Christ, but he will go before the Christ.
Thus begins the catechism according to Luke, a series of questions in order that we may have certainty concerning the things we have been taught.