The term"Christian" gets used to cover a wide range of different beliefs, all claiming to be based on the teaching of Jesus Christ. Christianity.net.au represents what is called"evangelical" Christianity. We think it is Christianity as Jesus and his original followers taught and practiced it. But Catholics see Catholicism as"true" Christianity.
Catholics and evangelicals believe a lot of the same things but the differences are very important.
Let’s start with some of the similarities. Both groups believe that there is only one God, who is three-in-one ("Trinity"): Father, Son and Holy Spirit. And both agree that Jesus is God the Son in human flesh.
They also agree that God is rightly angry with people for not treating him or other people as they should. After we die God will bring everybody back to life and then judge them, and the judgement we deserve is to be punished forever because of the way we’ve lived. But both Catholics and evangelicals believe that Jesus came to earth to sort out that problem by paying the punishment for us when he died on the Cross, and by coming back to life two days later.
Those are some of the similarities. One of the most important disagreements is about how we can actually be accepted by God. Evangelicals believe that if someone has faith in Jesus - if they trust him with their lives and follow him - then they are made totally acceptable to God because of Jesus’ death (Romans 3:23-26). The Bible says that this happens"apart from works" (Romans 4:6). In other words, it doesn’t depend on how good a life we live but only on our trust in Jesus. If we follow Jesus, we are guaranteed a place in Heaven when we die (Luke 23:43). Catholics, on the other hand, believe that our acceptance by God does depend partly on the life that we’ve lived. If we’re not good enough, we won’t make it to Heaven.
Another big difference is that evangelicals believe that it’s in the Bible that God speaks to us to tell us about himself and how he wants us to live. Catholics believe that he also speaks through the teachings of the Catholic church and the Pope. But Jesus said that only the Bible is the word of God, not the additional teachings of religious leaders (Mark 7:1-13).
I hope that’s some help. There’s a bit more on this topic here: http://www.christianity.net.au/questions/catholicism_and_christianity