We know that Christ came to redeem us because we could not redeem ourselves. Although it was we who committed the sin... we were not only unable, but unworthy to redeem ourselves. Why is this? It is because we had fallen so far from God, and were so blemished ourselves, that we could not even approach God to pay our own debt. While Christ could offer a pure and absolute sacrifice.
Some people wonder why evil exists... if the world began with God, who is all good, He must have made evil, right? Wrong, although this is the most logical conclusion. The reason evil exists is because God is good. In fact, goodness is defined through and by God. What is of God, is good. What is against God, is evil. Therefore Divine Justice also exists because God is good. Evil cannot pass into His sanctuary because His Good is entirely pure. Which is why we must be purified in purgatory before even stepping before His gaze if we have died with blemishes on our soul.
The angels first, and now we, are capable of committing evil because God recognizes, in His supreme wisdom, that we cannot really be good ourselves unless we have risen out of a slavish, animal attachment to His goodness and reached a living goodness. This obviously could only be done through the life of our souls, a personal identity that makes us children instead of plants, and thus we must have personal choice. Indeed, God's goodness is also a love which wills us to be godly children and heirs! Therefore, our goodness must be living and capable of choice if it is to reach the living perfection He desires us to be. Consider also what this means that Jesus did. By taking our human form He allowed us to share in His nature through brotherhood, giving us a bridge that we can cross with Him, becoming nothing ourselves and everything through Him, to reach this pure goodness which only God's eternal choice, His BEING of goodness, could have obtained alone.
When Jesus came into the world, He made us clean through His sacrifice. Through Him we are healed and made worthy to approach the Beatific Vision. However, He did not remove our free will and He did not come to eliminate evil from the world. Therefore, there not only remains our free will, but also the evil that causes us to fall time and again. Yes, we have dirtied ourselves after His sacrifice has made us clean, and therefore we must be clean again.
When we have died, and are still darkened by the evils of sin, we must be purged in purgatory so that we can be pure enough to enter Heaven and step into the presence of good. However, while we remain on earth we retain the ability to utilize Christ's sacrifice! Yes, even after we have dirtied ourselves again, His perpetual Sacrifice continues to clean us so that we can start anew. The sacraments of Penance and the Eucharist can be received over and over to cultivate cleanliness within us.
At the same time, the Heavenly justice is constantly being appeased for our sake. First and foremost this is done through the Sacrifice of the Mass, but it is also by the individual purification of people throughout the Church. A woman in a cloister, for instance, may be in constant prayer and purification through penance, and the merits of these are offered with the Holy Sacrifice for the sake of the Church. Each of us helps to purify the Church through our personal purification, through the merit of our One Bread and One Body. Thus our payment of the debt rises out of the individual level to the thorough and absolute level obtained by Christ's Sacrifice. A nun prays for those who will not pray, and her efforts are accepted by God and the graces are redirected at the Church's (and all its members) needs.
One day the good will be with the good, and be separated from evil forever. We must seek always to receive Christ's help so that we too can achieve the living goodness of heaven. Although we cannot, of ourselves, achieve this... we must have faith in Christ's Sacrifice and the Supreme Goodness who calls us to be His Children. If we have faith in this, and choose to be good, we shall be lifted up and placed on a seat of purity which will bring us everlasting joy.