The First Talk
The first talk (or sermon) given in the Church was by Peter the Apostle on the Day of Pentecost, immediately after he and others had been filled with the Holy Spirit.
A crowd had gathered because they heard them speak with tongues.
This talk from Pr Geoff Beggs covers Acts 1 and 2, including the period before the first talk, the period after the first talk, and the first talk itself.
The first talk of the Church is surely momentous and worth considering.