Thanks to the Protestant Reformation, Christianity has now grown into a behemoth of tens of thousands of differing Christian denominations. When one divorces himself from obedience to a central teaching authority, or magisterium, one opens himself up to being his own arbiter of what it means to be a Christian. "I don't believe [A] even though that's what my church teaches" leads to a separation from this particular body of beliefs...and one cannot then deny another fellow brother in Christ the right to say, "Yeah, well I don't believe [B] even though you believe that's what is required to be a good Christian".
It's just what the devil ordered, IMHO. Chaos and confusion regarding what it means to be a Christian. Separation and disunity on doctrine. Is baptism required for salvation? Is Saturday the Lord's Day? Can we lose our salvation? Are we going to be Raptured? Is God a Trinity?
Are the above "essential" to being a Christian? Since the Bible doesn't tell us what's an "essential" doctrine, it depends upon which Christian you ask.
It will be posited, however, that Catholicism is not immune to these divergent theological opinions also. The "Tu Quoque" ("but you do it, too!) argument is presented.
And they are correct: there is indeed disagreement among Catholic circles as well. Should girls be permitted to be altar servers? How can All Saints Day be a Holy Day of Obligation in one diocese but not in another diocese? (This is based on the bishop's discretion). Why do some diocese get a dispensation during Lent for St. Patrick's Day to eat meat? (Also based on the local ordinary's discernment). Which is better--that Latin Mass or Mass in the vernacular?
Apologist Jimmy Akin gives a response: "A number of points may be made: First, Catholicism has a functioning magisterium that can decide that these matters are not essential differences. Second, the relevant schools adhere to the teachings of the magisterium and, if their views were reprobated, would accept the results (or cease to be faithful Catholics). Third, the differences between Catholic schools of thought have nowhere near the magnitude of the difference among Protestant schools. Compared to the differences among Protestant groups, differences among orthodox Catholic groups are trivial. Finally, the fact that the Catholic Church has a magisterium means that there can be—and on the most important theological matters there is—an official Catholic position. There is no parallel standard in Protestant circles that can speak for Protestantism."
And a Protestant may also offer this objection: you all have your own problems in your Catholic churches. Look at all the Catholics who disagree with your Church on matters like abortion, homosexuality, women's ordination.
We respond: this is not an equivalent comparison. Catholics who disagree with Catholic teaching are being "bad" Catholics. However, Protestants, when they diverge from their own church's doctrines, are simply being "good" Protestants...following their own personal, private interpretations of the Word of God, as has been permitted and celebrated by Protestantism since the Protestant Reformation.