The excessive heresy found in the Catholic Church leads many people to ask "are Catholics Christians?" The answer will depend largely on who you ask. With exceptions, most Catholics will say "of course we're Christians!", with some even claiming that if you're not Catholic, you're not Christian. On the other side, many Protestants will say that it is impossible to so much as identify as Catholic and remain Christian. Even the very inclusion of the word "catholic" in the Nicene Creed is troubling to those who mistakenly believe it is naming the denomination (when in reality, it simply means "universal"; a fully committed Christian should have little trouble affirming the Nicene Creed).
Unfortunately, the Catholic Church itself is a lost cause, as it explicitly denies the Gospel. In his letter to the Galatians, Paul addresses a seemingly apostate group, not even attaching his usual epithets of honor in his introduction, but merely addressing them as "the churches in Galatia" (Galatians 1:2). The primary cause of his letter is to rebuke them for accepting "...a different gospel, which is not another;..." (v6-7). He goes on to say "But even if we, or an angel from heaven, preach any other gospel to you than what we have preached to you, let him be accursed. As we have said before, so now I say again, if anyone preaches any other gospel to you than what you have received, let him be accursed." (v8-9).
This makes it perfectly clear, both to the Galatians and to all of us; not even an Apostle has the right to teach a different gospel, and indeed any other gospel is no gospel at all, that Paul had not already preached in his capacity as an Apostle, selected by Christ for that very purpose. Anyone, even if they are an angel, an Apostle, or a fake Church claiming Apostolic authority 2,000 years later, who preaches a different gospel, according to Paul, is accursed. The problem? The Catholic Church does teach a different gospel, along with a host of other unbiblical doctrines.
But whether or not Catholics are Christians is a different matter entirely. Like most religions, Catholicism contains a large number of people who do not know, or do not readily affirm, its doctrines. There are certainly many, such as apologists Trent Horn and William Albrecht, who cannot be considered Christians because they teach the false gospel of Rome, and thus, according to Paul, are considered anathema. But there are many Catholics who know so little of Rome's doctrines, you can quote the Catechism of the Catholic Church at them, they will still deny that either they believe it, or even that their Church teaches it.
Over the centuries, the Roman Catholic Church has proven itself to be a lot more Roman than catholic, having "Christianised" a number of pagan beliefs. Even the Pope, who goes by the title "Pontifex Maximus" (supreme pontiff) stole that title from the chief priest of the Roman state religion. No one should see this as a mere coincidence. Over the centuries, it has evolved into a toxic milkshake of Christianity, and a myriad of false religions the Catholic Church absorbed over the centuries (of course, with several other flawed interpretations of scripture mixed in).
But Christianity is still the "milk" of this shake. Indeed, while Pope Pius IX said "All our hope do we repose in the most Blessed Virgin..." (1), Catholicism still affirms the birth, death, and resurrection of the Lord Jesus Christ, the Son of God, even accepting the doctrine of the Trinity. This to the point where many Catholics, reading the aforementioned papal quote, will recoil, and may even accuse me of having fabricated the quote. It's that obviously blasphemous, and if you read on, it gets far, far worse. And many Catholics, out of love for Jesus, will recognise that.
And so there really is no more simple an answer to the question "are Catholics Christians?" than "are Evangelicals Christians?" It is, and will always be, dependent entirely upon the individual. Do you affirm the Gospel Paul preached, or not? If the answer is yes, then the only real way to discount you as a Christian is if you simultaneously reject it. If you accept it, yet still call yourself Catholic, you're certainly compromised, but not enough to say you're not Christian. If you reject the Gospel, Paul says you're anathema, no ifs ands or buts. So, are Catholics Christians? Maybe. This Catholic is, that Catholic isn't. I generally default to accepting a person's profession of faith, especially if their fruits match. But if someone departs from the Gospel, they have departed from Christianity.
References
1. Apostolic Constitution of Pope Pius IX on the Immaculate Conception - December 8, 1854 (link).