As he stood in this synagogue, Paul proclaimed the basic Christian gospel. First he proclaimed that Jesus was the Savior of the world:
From this man's seed, according to the promise, God raised up for Israel a Savior - Jesus - Acts 13:23
Next, Paul taught that taught that after Christ's death fulfilled the demands of Scripture, Jesus was raised from the dead:
And though they found no cause for death in Him, they asked Pilate that He should be put to death. Now when they had fulfilled all that was written concerning Him, they took Him down from the tree and laid Him in a tomb. But God raised Him from the dead.
Acts 13:28-30
Next, Paul taught that taught that Jesus was the Son of God:
As it is also written in the second Psalm: "You are My Son. Today I have begotten You.
Acts 13:33
Finally, Paul taught that one is "justified" (declared "not guilty" of their sins) as a result of faith in Christ, and not by the works of the law:
Therefore let it be known to you, brethren, that through this Man is preached to you the forgiveness of sins; and by Him everyone who believes is justified from all things from which you could not be justified by the law of Moses. Acts 13:38-39
To be justified apart from the law of Moses is to be justified by God's grace. Here, in the province of Galatia, the battle for grace began.