The difficulty with answering this question is that the Catholic church is such a big organisation that you will find people and churches within it who believe all sorts of different things. So in saying this is what the catholic church believes, I am actually making a generalisation for which there will be exceptions. When talking about this with a Catholic it is always best to ask them what they believe rather than assume they believe what the catholic church believes. Given that, I can make comments about the official stance of the Catholic church and that is what this answer is based on.
Basically, the difference boils down to how Catholicism and Christianity answer this question - ‘How can people have a relationship with God?’ The Bible says we’ve all put ourselves in charge of our own lives when really God should be in charge. That’s rebellion, or sin, and we all deserve to be punished for it. But how can the relationship be fixed so that instead of being punished we can have a good relationship with our creator?
The Catholic church says there are two things you need. Firstly, you need the Catholic church. They would say that by being a part of their church and doing what it requires you’ll be changed enough to be acceptable to God. The other thing you need is you. You need to do enough good things to be acceptable to God. By being part of the church and doing lots of good things you can earn enough good merit for God to be happy with you.
The Bible says something completely different. The Bible says we’re dead in sin (Ephesians 2 verse 1). Dead people can’t help themselves, can they? We’re so stuck in rebellion against God that there’s nothing we can do to save ourselves. We’re guilty rebels and we deserve a rebels death. But the Bible also says that God loves us so much that he wanted to rescue us. He sent his own son, Jesus, to help us. Jesus never rebelled, so he didn’t deserve punishment. But he chose to die in our place, instead of us. It was a swap. He died so we can go free. The Bible says ‘For Christ died for sins once for all, the righteous for the unrighteous, to bring you to God’ (1 Peter 3 verse 18). Then God raised Jesus to life as the king of the world. Now, instead of trying to work our way into God’s good books (which we can’t), we simply have to trust the king he sent to fix the problem. We ask God for free forgiveness and we trust that Jesus’ death on our behalf worked. We also need to submit to Jesus as king and do the good stuff he wants us to do. This is a response to what he’s done for us, not an attempt to earn God’s favour.
If God could find any way of dealing with our sin that was still just and fair, other than letting his Son die, he would have done it. But Jesus is the only way to be forgiven and have a relationship with God. Will you trust God’s good king?