To begin, I should clarify exactly what a Christian is, or what it means to be in Christ. A Christian is someone who has repented from their sin and has believed in Jesus. The Bible teaches that this also needs to entail an understanding that Jesus is Lord.
Romans 10:9 says,“If you confess with your mouth, ‘Jesus is Lord,’ and believe in your heart that God raised him from the dead you will be saved.”
Jesus’ message was,“The Kingdom of God is near. Repent and believe the good news!” (Mark 1:15).
Being a Christian has nothing to do with the church that I choose to go to or the denomination that I choose to align with. Being a Christian has to do with acknowledgment about who Jesus is and turning away from sin and back to him.
So the question that I want to ask anybody who claims to be a Christian is have they truly repented from their sin and turned to Jesus? Let me also say that there are some church-going Anglicans who sadly have not truly believed in Jesus and who align themselves with the Anglican Church or attend church for a whole host of other reasons.
As for the question of doctrine, you are right that the Catholics have made some serious errors in their understanding of the Bible. The most serious of these has to do with the very question above, what is a Christian. The error that has been made is that a person becomes acceptable to God by what they do rather than by what Jesus has done on the cross. This is what we call being saved by ‘works’. The problem is that no-one is good enough for that, that’s why Jesus went to the cross to die for sin.
But there are many Catholics who have truly repented from their sin and turned to Jesus in faith. That means that they are also Christian brothers and sisters.
So I would recommend a Christian to not go to a Roman Catholic church because of what they teach, but if they do it doesn’t mean that they’re not a Christian.