It was the atonement for all the sins of the world all at once by believing in Jesus. Since Jesus is the Son of God and our Lord who lives forever, He can take away all the sins of the world forever. How did He take away our sins forever?

He did it
(1) by being born in the flesh of a man,
(2) by being baptized by John the Baptist at the Jordan,
(3) by being crucified on the Cross to take all the judgment in our stead,

The Son of God came to this world in the flesh of a man and was baptized to take away all the sins of the world through John the Baptist. He then bled on the Cross to save humankind from all their sins forever (Leviticus 16:6-22, Matthew 3:13-17, John 1:29, Hebrews 9:12, 10:1-18).