For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
Catholics are Christians because they believe and confess Christ. They believe in his resurrection, the apostle's creed tells it all. Romans 10:9 "That if thou shall confess with thy mouth The Lord Jesus and shalt believe in thine heart that God hath raised him from the dead, thou shall be saved". Their main issue is they are still under the law practicing the law when grace is available.... Romans 6:14 "For sin shall not have dominion over you: for ye are not under the law, but under grace.
Their main issue has nothing to do with being under or practicing the law, but rather the following:
1. Catholics view the Bible as having equal authority with the Roman Catholic Church and tradition, whereas Christians view the Bible as the supreme authority for faith and practice.
2. To Catholics, saints are a special group of holy people who are venerated. These people may act as intercessors between God and Man, and may also be called upon in prayer. The Bible teaches that all Christians are saints and Jesus is the only mediator between God and Man.
3. Purgatory is a place Catholics believe they will go if they die with unpaid penalties and there they will pay for their penalties by suffering. The Bible does not support this view (Hebrews 9:27 NLT “And as it is appointed for men to die once, but after this the judgment”).
4. Confession. Catholics believe that priests have the power and authority to forgive sins, therefore sins must be confessed to their priest. The Bible says Jesus has the power to forgive sins (Matthew 9:6). Also, no human priest is a mediator between God and man. The Bible also teaches we can ask God ourselves for forgiveness, and He promises to forgive us and cleanse us (1 John 1:9).
There are many other issues such as their beliefs of the assumption of Mary, the infallibility of the Pope, penance, priests not being able to marry, etc all of which are not supported by the Bible.