“Why so many different sermons? Does the message matter in this purpose?”
Just because those who are “ordained to eternal life, believe” (Acts 13:48) doesn’t mean that they all come to saving faith in the same exact way. The illustrious Matthew Henry, in his commentary on John, tells the story of a young man who was wonderfully changed upon merely reading the words “In the beginning was the Word, and the Word was with God, and the Word was God,” despite a lifetime of sin. Paul was converted on the road to Damascus. Those Gentiles in Acts 13:48 heard the Gospel through the particular preaching of the Apostles. How exactly we are brought, and how we are brought, and under what circumstances, are all in the hands of the God who “worketh all things after the counsel of His own will (Eph 1:11).
Wouldn’t the elect be the elect no matter what the sermon?