Absolutely all sins, from the past, present, and future, were forgiven that day on the cross.
Our Lord is an eternal now. He exists outside the realm of time, since time is a creation of God, and His actions can apply for all eternity. We have to work out salvation through time. Therefore we need the sacraments, especially confession, which applies the saving balm of the blood of Christ to our wounded souls.
Our divine Lord in His infinite wisdom and knowledge knew that we would deal with the residue of original sin, known as concupiscence. Though we are created good, and through baptism we have been redeemed by the blood of Christ, we still have the use of our free will and can freely reject these gifts from God. We may choose not to cooperate with the abundance of grace God offers to us through the sacraments. Christ, therefore, instituted the sacrament of penance as a vehicle of mercy and healing. In Mark 2:17, Jesus states, “…I have come to call sinners, not the self-righteous.”
Knowing that we have a weakened will, a darkened intellect, and passions that become inordinate at times, He understood that we would fall into sin and would need His forgiveness.
Classic scriptural reference to the institution of this sacrament of forgiveness is in Matthew 16: 13–19. In this passage, our Lord gives authority to the apostles and their successors, bishops, and priests of the keys of the kingdom. In fact, one of the most ancient symbols to denote the sacrament of Penance is two keys lying over each other. It refers to the binding and loosening. “I will entrust to you the keys of the kingdom of heaven. Whatever you declare bound on earth shall be bound in heaven; whatever you declare loosed on earth shall be loosed in heaven” (Matthew 16:19).
In fact, the priest who acts in the Person of Christ has to use this binding and loosening. Christ loosens with the key of forgiveness but binds in the case when the person is unrepentant and has no intention of changing. The priest, in the sacred tribunal of the sacrament, has to judge the intentions of the penitent. While it is rare that he has to defer the sacrament and withhold absolution, it does happen.
If a person confesses the sin of contraception and has no intention of changing their life because it’s too hard or they just don’t want to, a priest must withhold absolution until he can ascertain that the penitent is sorry and wants to change.