When I use the word “Christian” I use it to mean something like “a follower of Jesus” - which is what it seems to have originally meant - someone who has turned back to God (repented) and now lives with Jesus as their “master” and saviour.
The Catholic church is a human organisation which has its origins in the early gathering of Jesus’ followers (Christians) in the first century AD. Like all human organisations it is imperfect, and over time political and other motivations have led it further and further away from its original foundation.
+ The central message of the bible is that God is able and willing to forgive guilty people who turn back to him (read Luke 15:11-32 to see an example of this). The only reason this is possible is because Jesus shed his blood on the cross, paying the price for our human rebellion against God (e.g. 1 Peter 2:24 or Isaiah 53:5-6 - prophecy 800 years before the event).
+ The Catholic church generally obscures this simple message. This becomes obvious if you ask a Catholic whether they are going to heaven. A Catholic believes they have to reach a certain moral standard, and have to follow all the church rules in order to get into heaven, and the average Catholic feels inadequate about their performance. A Christian, however has Christ’s promise, John 3:36, “whoever believes in the Son has eternal life”, see also John 1:10-13 - being a Christian is simply about recognising who Jesus is and responding to him.
+ A lot of things have built up in the Catholic church, things like: purgatory, making Mary into a God, doing penance (saying “Hail Mary’s” etc.), saints, and a whole mass of superstitions and rituals.
+ The “Church”/Pope etc. is seen to have the ultimate authority, with the bible as a kind of reference manual. In Protestant churches, however, the bible is used (ideally) as the yardstick against which everything is measured, and is the only ultimate authority.