No, Roman Catholicism and Christianity are not the same thing. Christianity is properly defined by certain doctrines that are revealed in the Bible. It is not defined by simply saying that as long as you believe in Jesus, you're a Christian. Mormons believe in Jesus, but their Jesus is a brother of the devil in the pre-existence. The Jesus of the Jehovah's Witnesses is Michael the Archangel. So, just saying you believe in Jesus doesn't make you a Christian. This is why the Bible tells us who Jesus really is--God in flesh, creator of the universe. Likewise, there are essential doctrines; and if any of those essential doctrines are violated, then a church would only appear to be Christian but not really be Christian. What are those doctrines? The Bible tells us. Let's take a look.
- There is only one God, and you are to serve no other gods (Exodus 20:3; Isaiah 43:10; 44:6, 8).
- Jesus is both God and man (John 1:1, 14; 8:24; Col. 2:9; 1 John 4:1-4).
- Jesus rose from the dead physically (John 2:19-21; 1 Cor. 15:14).
- Salvation is by grace through faith (Rom. 5:1; Eph. 2:8-9; Gal. 3:1-2; 5:1-4).
- The gospel is the death, burial, and resurrection of Jesus according to the scriptures (1 Cor. 15:1-4; Gal. 1:8-9).
- God is a Trinity (Matt. 28:19; 1 Cor. 12:4-6; 2 Cor. 13:14).
- Jesus was born of the virgin Mary (Matt. 1:25).
So, someone who is a true Christian will believe these things and not violate them. Roman Catholicism violates two of them (#1 and #4). First of all, by its practice of promoting Mary (and the Saints) to the level of God-like capabilities, they break the commandment to have no other gods before the true and living God. In Roman Catholicism, they say that Mary is the mediatrix (Catechism of the Catholic Church paragraph 969); Mary made atonement for the sins of man (Fundamentals of Catholic Dogma, page 213); Mary is the subject of preaching and worship (Vatican Council II, p. 420); etc.
Also, Catholicism violates the biblical doctrine of salvation by grace through faith alone. Paul the apostle, for example, tells us in Romans 4:5, "But to the one who does not work, but believes in Him who justifies the ungodly, his faith is reckoned as righteousness." But Roman Catholicism denies that and says, " . . . so that all men may attain salvation through faith, Baptism and the observance of the Commandments," (CCC, par 2068). Also, consider this:
"If any one saith, that by faith alone the impious is justified; in such wise as to mean, that nothing else is required to co-operate in order to the obtaining the grace of Justification, and that it is not in any way necessary, that he be prepared and disposed by the movement of his own will; let him be anathema" (Council of Trent, Canons on Justification, Canon 9).
So, even though Roman Catholicism claims to be Christian and that it is the one true church, it violates the essentials of the Christian faith. It goes beyond what is written in God's word (1 Cor. 4:6). It denies the sole and true sovereignty of the living God by promoting prayer to and the worship of Mary. Also, it denies justification by faith alone in Christ alone. It is not a Christian church.