I was once taught that there are three levels of theological error - technical error, serious error, and fatal error. (These categories may not be the most helpful, but we'll use them for the moment.) So for example, someone who confuses God's aseity with His ubiquity is making a technical error that is simple and definitional with little or no consequence. However, someone who believes that drinking alcohol is always immoral for everyone is in serious danger of being unnecessarily divisive and legalistic. It is theological error with serious consequences.
Unfortunately, distinguishing serious error from fatal error is really tricky. How much theological error is too much? Can you reject God's immutability in favor of "process theology" and be a Christian? Serious or fatal? How about a mechanistic understanding of the sacraments? Serious or fatal? How about being a universalist? Serious or fatal? The list goes on and on.
In my dotage I am much more reticent to identify fatal error in someone who heartily affirms the Trinitarian and historical-Biblical-theological tenents of the Nicene Creed - even if they aren't familiar with the creed(s) at all. Heck, I think you could potentially strip-down what it takes to be a Christian even more. Hebrews 11:6 reads, "...because anyone who comes to him must believe that he exists and that he rewards those who earnestly seek him." Now clearly, Hebrews teaches a robust understanding of Christianity and the kind of faith that it entails. And yet... faith is simple and should not be overly complicated with theological nuance. Yes, we should pursue orthodoxy and Christian maturity, but we should be very cautious about citing fatal error in those who are simply immature and / or ill-taught.
I once had the opportunity to spend three hours with the Reformed theologian, John Gerstner. I asked him, "Is believing in the tenents of the Apostles' Creed enough to be a Christian?" He became very animated and said, "No! Not anymore!" He went on to say that the early creeds only dealt with controversies of the early church - there have been many important issues since then that simply must be affirmed and / or denied. I agree that there have been many issues that need to be wrestled with, but I would put almost all of them in the category of serious error and not fatal error.
Perhaps the reason for my reticence is that I am fearful of my own error(s) - and trusting and hoping for mercy now and in judgment. I willl be shown mercy as I show mercy, so I am in no hurry to condemn error as fatal if there is any hope or room for diagnosing serious error.
Is this not the practical outworking of "sola gratia?" How can those who hold to this be so quick to cite errors as fatal? Perhaps I am just a liberal afterall...