What a truly dangerous sentiment. To suggest that Calixtus was able to, and had the authority to, abrogate God's word, and the holy teaching of Christ and His apostles, is simply unacceptable.
I reject the idea that the New Testament forbids post-baptismal forgiveness. The apostolic word is the bounty and riches of God's forgiveness, not just as a one time moment, but as an everflowing spring without end. And so St. John writes that if we confess our sins God is faithful and just to forgive us our sins, it is why the St. Paul calls the Faithful to repentance, it is why in the Apocalypse Jesus says, "Behold, I stand at the door and knock", He invites the churches to remember their first love. Time and against we have this.
So no, the New Testament does not forbid forgiveness of post-baptismal sins.
It seems strange that one would appeal to Tertullian, who abandoned the orthodox faith for heresy, to further support such a position.
That there is forgiveness from God, in Christ, for all our sins is apostolic and catholic teaching. If it is an innovation from Calixtus, such would utterly undermine the preaching of the Gospel, and the Church herself.
-CryptoLutheran