Election and salvation are not the same.
What makes sure our salvation is believing in Christ’s shed blood on the cross for our sins. Someone could never do one “good deed” and still be in heaven upon death. Sadly, they would have very little as far as rewards at the Judgment Seat of Christ.
Well, the difference is that apparently the election still needs to be secured, before it becomes salvation.
What makes it sure, St. Peter explains in the verses that I posted. It only starts with faith.