God created mankind in His image- Holy. Holiness is the ability to balance love and justice for the good of the other person. But when Adam sinned, his holiness was replaced by sinful selfishness. Not only was mankind's nature changed, but his relationship with God and others was hurt. Rather than being friends, Adam hid from God. Rather than being partners and equals, Adam and Eve shifted to hyper-competition and shame and blame. Their sinful nature caused them to act sinfully.
This is our two-sided problem. First, we inherited Adam’s sinful nature. Second, our sinful nature causes us to commit acts of sin. That’s a problem because, in God’s justice, He demands sin to be paid for with our blood, our lives.
God sent his only son, Jesus, to show us how to have a right relationship with each other and God. Then God allowed him to die as a substitute for us. He didn't have any sin of his own to pay for. So, his death gave him the authority to pay the price for our sin. In order to accept this gift of forgiveness, He requires us to commit ourselves to following Him, putting his teachings into practice. This solves the problem of our punishment.
However, we still have a sinful nature that causes us to rebel against God. If it isn't dealt with, we will constantly sin against God and behave selfishly in our relationships. So, God sent the Holy Spirit to live inside our body and to give us power to resist the sinful nature's selfishness. When we ask Jesus to forgive us, he not only forgives our sin, but also puts the Holy Spirit inside of us. The Holy Spirit gives us the ability to say "yes" to God and "no" to sin. He also gives us the ability to love God and love people. He restores God's holiness in our character.