From the way that I understand Penance is that one does not have to go to Confession to confess venial sin only mortal sin. It is recommended to go frequently even if you are not under the state of mortal sin to receive the inherent graces granted from the sacrament.
Mortal sin can be forgiven outside of confession if the individual is perfectly contrite for the sin. But how can an individual trully know if he is perfectly contrite? Confession insures forgiveness.
Also it is required by Christ because sin just does not impact the sinner. It impacts the community as well. So one not only needs to be forgiven by God but he/she needs to also be forgiven by the community (Church) and even one’s self.
In other words the Calvinistic way is imperfect and is not a guaranteed method of forgiveness. It is extremely individualistic and does not emphasized the exterior influences of sin in the community.
God’s graces are freely given to everyone who participates in mass. That is very different in saying that God’s graces are universally received by everyone who particates in mass. We all make decisions to receive God’s graces or not and that is what you see. When I go to mass I believe I am being changed, progressing in holiness, desiring God more, I just can’t attest to anyone else.
I agree that our Protestant brothers have the Spirit of God working in their lives and usually when they come over to the Catholic church they are usually some of the more outspoken Catholics. If Catholics only accepted what the Church offers to the point that many Protestant and even Mormans do, there would be a fire burning so bright around our parishes that there would be no need for this conversation for all Christians would be Catholics. Let us pray that one day this sleeping giant awakes and makes it so.