2 Corinthians 7

As one considers the teaching of Paul -

"Knowing that a man is not justified by the works of the law, but by the faith of Jesus Christ, even we have believed in Jesus Christ, that we might be justified by the faith of Christ, and not by the works of the law: for by the works of the law shall no flesh be justified." (New Testament | Galatians 2:16)

One must also consider this teaching -

"Now I rejoice, not that ye were made sorry, but that ye sorrowed to repentance: for ye were made sorry after a godly manner...
10 For godly sorrow worketh repentance to salvation not to be repented of: but the sorrow of the world worketh death." (New Testament | 2 Corinthians 7:9 - 10)

It is Godly sorrow that worketh repentance unto salvation. Combining both verses, one must believe that we are justified as we have faith in Christ and truly repent of our sins. For as we do we become a new creature, or His sons and daughters -

" For behold this selfsame thing, that ye sorrowed after a godly sort [God's way], what carefulness [sincerely] it wrought in you, yea, what clearing [cleansing] of yourselves, yea, what indignation [regret sin], yea, what fear [recognizing the justice of God], yea, what vehement desire [to change ones heart], yea, what zeal, yea, what revenge [recognition of potential punishment]! In all things ye have approved yourselves to be clear in this matter." (New Testament | 2 Corinthians 7:11)

I believe these principles with the ordinances of baptism and receiving the Holy Ghost, is the very doctrine of Christ.