1 John 1:9

If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness.

(v9, NIV)

Why is God faithful and just to forgive us our sins? First of all, if we sin, justice demands us to be punished. The punishment for our sins is death. We are dirty and unclean due to our sins. We cannot come into the presence of a holy and righteous God when we are clothed in our filthy sins. God's justice demands punishment for our sins. We totally and entirely deserve to die and be sent straight to hell. However, though we were on a path of destruction, God intervened. God made a way for us to be saved. Jesus Christ came down in the flesh as a man and died, paying the penalty for our sins. Jesus' righteousness is placed on us and our sins are placed on him. Therefore, God's wrath for the sins we committed was satisfied by the death of Jesus Christ on the cross. As a result, God's justice is satisfied and we are allowed to come into fellowship with God as his adopted sons and daughters. We are saved by grace through faith in Jesus Christ. The faithfulness John is referring to is God's faithfulness to keep to his promises. We need to repent and turn away from our sins, and God promises to forgive us. God is just because the payment for your sins has already been paid for. It would be unjust for God to make you pay for them again if you believe unto Jesus and allow him to pay for your sins. You need to confess your sins and turn away from them. Similarly, if someone paid for my meal, it would be unjust for the restaurant to still charge me even though someone already paid for it. God is just, and he will not force you to pay if you accept Jesus' free gift, paying for you. Confess your sins, God will forgive you. However, this must be a genuine repentance and turning from your sins. God knows if you're genuine or not, you can't fool him. But if it is genuine, God will forgive you and cleanse you. He is faithful and just.