If the Church is to fulfill in its entirely her task of saving mankind she needs the power to forgive sins. It is a power essentially different from her mission to preach the Gospel and baptize. In baptism, indeed all sins and the punishment due to them are remitted. Baptism is the first justification. But the first justification is also the first entry into the realm of the supernatural which works entirely by God's grace and which asks of the person baptized no more than that he turn away from sin and turn in faith to Christ.
Penance is something different.
A baptized person who sins again, sins against God to whom, since his baptism in the name of the Most Holy Trinity, he belongs. He also betrays the Church of which he is now a member. Thus, the new atonement assumes the character of a legal trial, with accusation, sentence and satisfaction. The practice of penance has varied considerably down the centuries. In very early days satisfaction, usually in the form of public penance, was very much to the fore.