I've heard a handful of Christians I've talked to online refer to Catholics as non-Christians. I'm not really sure why, and I'm usually skeptical when I hear a group who professes Jesus Christ be called a non-Christian sect because I've even heard Old Earth Creationists like Hugh Ross and myself be labeled as non-Christians because we deny the days of creation as 6 24 hour periods and we believe that they're long periods of time. Arminians likewise have been labeled as non-Christians by over zealous Calvinists (though most Calvinists I've talked to don't hold the view that non-Calvinists are non-Christians). This is why I always want to investigate what the said sect believes before I form my opinion on whether the group in question is really Christian or not. Because often times a group is called non-Christian merely because the person disagrees with them on some secondary issue. Usually what makes a group non-Christian is that they deny some or many of the most crucial, central doctrines of the Christian faith (doctrines such as The Trinity, the deity of Christ, the resurrection, the virgin birth, or that unbelieving sinners go to Hell, or that Christ is the only way to Heaven). Jehovah's Witnesses for example, deny the deity of Christ (which, usually, the deity of Christ and the trinity go hand-in-hand, if you accept or deny one, you accept or deny the other). Jehovah's Witnesses believe that Jesus is St. Michael, not God in the flesh.
But what about Roman Catholics? Are they Christians or not? First, what do Catholics believe? I did some studying into Catholicism a few years ago (spending time watching EWTN and the "Crash Course In Catholicism" videos), right around the time I became a Christian. I was Catholic leaning for a while there, though as I've studied scripture and reflected on the issues which separate Catholics from non-Catholics, I think I'm a firmly established Evangelical.
I've discovered that Catholics and I agree on a lot of things. We both agree for example what's stated in "The Apostles Creed".
"I believe in God the Father Almighty, Maker of heaven and earth: And in Jesus Christ his only Son our Lord, He was conceived by the Holy Ghost, born of the Virgin Mary, suffered under Pontius Pilate, was crucified, dead, and buried: He descended to the dead, On the third day he rose again from the dead; He ascended into heaven, and is seated at the right hand of God the Father Almighty; He will come to judge the living and the dead and His kingdom will be without end, I believe in the Holy Ghost; The holy Catholic Church; The Communion of Saints; The Forgiveness of sins; The Resurrection of the body, And the Life everlasting. Amen."
I'd like to point out that "Catholic" in this creed doesn't just mean ROMAN Catholic, rather, it's referring to the "universal" Church. "Catholic" means universal. Ken Samples does a good job of pointing this out in his commentary on the apostles creed in his book "A World Of Difference: Putting Christian Truth Claims To The Worldview Test". Moreover, "communion of saints" could be referring to Christians being ushered into the Kingdom of Heaven rather than special people ordained by the church after their deaths. Regardless, Evangelicals can clearly see from this creed alone that is recited by Catholics during the Divine Mercy prayer and during the Rosary prayer, that we share a lot of common ground. Notice also that the essentials of the Christian faith are mentioned in this creed. Belief in God as the Creator of the universe, belief in Jesus as God, belief in the virgin birth and the resurrection, and belief in The Trinity.
If Roman Catholics did deny of these things, I would consider them as much a non-Christian group as I do Christadelphians, Mormons, Jehovah's Witnesses, etc.
I don't think Catholics are non-Christians. I do think they hold some unbiblical doctrines (purgatory, confession to a priest etc.), but they seem to adhere to all of the essentials.
I don't agree with several things they belief in. I don't believe we go to Purgatory before we enter Heaven, I don't believe you need to confess your sins to a priest in order to be forgiven, you can go straight to God Himself (see 1 John 1:9, & Psalm 51 for example), and I don't believe that the communion wafers and wine LITERALLY turn into the body and blood of Christ (If they were literal flesh and blood, wouldn't they taste like skin and blood? Why does the Church insist that our senses are deceiving us?) In any case, I don't think having a few secondary issues wrong makes one a non-christian. I still believe they are my brothers and sisters in Christ, and I should like to think we can have fellowship with one another in spite of some theological disagreements just I get along with Calvinists and young earth creationists even though I disagree theologically with them. Trust me, I think the Calvinists are dead wrong on the things that separate them from their oppositions (Arminians and Molinism) and I also find Theistic Evolution to be inconsistent with scripture. Nevertheless, I still consider them my brothers and sisters in Christ. I think the Catholics are misguided on many things, but I don't think God will bar them access to Heaven on the basis of those things because I consider them to be just as secondary an issue as the age of the Earth or whether baptism should be full submersion or sprinkled.
And contrary to popular belief, the Catholics don't worship Mary, they just greatly admire her. Moreover, they don't pray TO saints. They have the saints intercess for them. It's no different than asking one of your friends to pray for you. It's just that in the case of the saints, the saints are are no longer living and they're in Heaven. These are two very common misrepresentations of Catholicism. Whether you agree with them or not, you should at least look into what they believe so you don't misrepresent their view (a fallacy known as "Straw Man").