Are All Hardshells Lost?

I have been asked this question several times through the years. Recently I had some talks with a sister over this question. Seeing they so clearly oppose scripture, teaching heresy, how can they be saved? Well, since then, I came across an old article of mine wherein I said the following:

"Hardshellism is a disease in the body of Christ. Hardshell teaching against the idea that one must believe in Jesus to be saved is serious. God is not pleased with such teaching. It is anti-gospel. God's word is "believe and be saved" (Acts 16: 31) but the Hardshell word is "be saved whether you believe or not." God's word says that the wrath of God abides on the unbeliever, but Hardshells believe many unbelievers are justified from such wrath. (John 3: 36) Now, I do not say that all Hardshells are damned, because many of them truly do believe in Jesus and in the gospel. But, that is not to say that God will not judge them severely for their preaching an anti-gospel." (From my article in The Baptist Gadfly Faith In The Blood)

That is still my view.

The question for discussion, with fear and trembling, is this: what errors in bible doctrine are eternally damning to believe? Or, how much error can a person believe and yet be saved? 

Today's Hardshells go to one extreme in affirming that one does not have to believe the truth on anything to be saved. They will argue this way in support of that view: infants who die in infancy and idiots without cognitive ability go to heaven and they did not believe in any truth or any error. So, it is the same way with those who are not infants or idiots. 

Some go to another extreme and argue that one must be right on nearly every bible doctrine to be saved. We see such people in some Calvinist groups and in some Arminian groups, such as the "Church of Christ" or "Campbellite" people. 

The following verses tell us that what we believe is important and that we should be on guard against heresy, which is a work of the flesh. (See Gal. 5: 19-23)

"...he that cometh to God must believe that he is, and that he is a rewarder of them that diligently seek him." (Heb. 11: 6)

"That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved." (Rom. 10: 9)

"Whosoever transgresseth, and abideth not in the doctrine of Christ, hath not God. He that abideth in the doctrine of Christ, he hath both the Father and the Son." (II John 9)