Explain Like I'm Five

Catholic here. Despite God's forgiveness, Jesus never said salvation is guaranteed. As he said, "it is easier for a camel to pass through the eye of a needle than for a rich man to enter the kingdom of Heaven". And that's just for the rich. There are other passages that emphasize the difficulty of gaining eternal life in Heaven, " the way is narrow", "you must take up your cross", and so on. Christ's death and resurrection made salvation possible in the first place. We couldn't even have a hope of it without His help. He also gave us the way that we must follow to gain salvation now that it's possible: belief in God and Christ, and following His commandments, given through the Church.

To put it in another way, we all have a relationship with God. That relationship was damaged through original sin in a way we could not repair on our own. God still has always loved us, but without Christ's sacrifice, He could not forgive our betrayal through sin and therefore we remained separated from Him. Once Christ bore the burden of our sin and overcame it, that repaired humanity's relationship with God overall and God is willing to forgive any sin, past or present, that we commit against Him. As long as we do not commit a serious sin, that relationship will stay intact. Two people in a relationship may do little things that annoy or lightly anger the other person, but we've all got stuff that aren't "deal-breakers" with each other. But a serious sin done with full knowledge and of one's own free will, which in the Catholic Church we call a mortal sin, is a "deal-breaker" that once again severs our own personal relationship with God and threatens our salvation. It's basically a betrayal of God's love. God has these rules and morality and such because He loves us so much He wants the absolute best for humanity and the world. Sin does damage to that, and mortal sin does damage to that in a big way. God is always willing to forgive, but in order for that to happen we have to show that we are sorry for breaking that relationship and promise/resolve that we will do our best to try not to do it again. We have to reconcile with God just as two people in a strained or broken relationship have to reconcile with each other. In the Catholic Church, we believe that reconciliation happens in the Sacrament of Reconciliation, where we confess what we did to a priest, who is in the person of Jesus at that time, and make that resolution to not sin again. The priest then offers a penance as a way to basically "make it up" to God, or as a theologian I heard once say, "clean up the glass and repair the window we broke", and the good relationship with God is restored. Basically, yeah, God is always willing to forgive if we ask for it... But that doesn't mean we still can't break that relationship. I'd always be willing to forgive a best friend if they were to betray me, but if they actually did that, I'd still be mad, and if they don't respond to my calls offering that forgiveness, well, there's not much more I can do to fix the relationship with my friend at that point if they don't want to be forgiven.