How is it that the death and resurrection of Christ allows for God to be both just and merciful
with humanity?
Love is an attribute of God. I think it is not possible to love without being just. God is unchanging. So He is always Holy. Corruption of sin is against His nature. We constantly sin. So, we leave Him only one option, to punish us. He knows our fallen nature and that the devil tricked Adam and Eve. He decided to take the sins on Himself. From His holiness, He taught us right and wrong. Scriptures are clear that transgressors will be punished. He died on the Cross to atone for the sins. He showed His faithfulness by resurrecting Himself. He has given us the Sacraments to be in union with Him.
"For God so loved the world that he gave his only Son, that whoever believes in him should not perish, but have eternal life. For God sent his Son into the world, not to condemn the world, but that the world might be saved through him" (JN. 3:16-17).
Very often, almost exclusively we look at this passage in the context of mercy, but there is justice in this passage because there is a requirement we must meet in order to obtain this mercy; that we "believe in Him". To believe in Jesus is more than just an acknowledgement of who Jesus is, the demons did that (Mt. 8:28-29), but it is to live as we are called to live by Jesus (Mt. 19:16-30, Mt. 25:31-46, James 2:17-18). Abraham was the first to be called to live this standard when he was tested by God in chapter 22.
We are not worthy of heaven, but God, in his love for us desires us to be with him. He creates this opportunity for us through his merciful act of sending his Son to redeem us. Because God is just there must also be some form of reconciliation on our part too; we need to show God that we wish to amend our lives to atone for our sin; our severing of our relationship with him and seek to live rightly. This is expressed in the story of the rich young man in Mt. 19, the parable of the Good Samaritan (Lk.10), the parable of the Prodigal son (Lk. 15), the lesson on forgiveness (Mt. 18:21-22) and in the judgment of nations in Mt. 25.
God, in his mercy gives us a way to go where we don't deserve to go, to heaven, but in his justice he requires we prove we desire to accept his mercy by our own free will.
