Indeed that is one aspect of the prevailing question/s. What is/are the outer boundary/ies or perimeter? Is it an ecumenist or syncretist doctrinal free-for-all with no accountability whatsoever for even Christology?
When the lawyer answered that the whole law was fulfilled with "Love God" and "Love thy neighbor" he countered with the question of "but who is my neighbor?" Jesus gave the more expansive and inclusive answer of even the Samaritan that passed by the way. If the question is still "who is my brother" then I think it is also appropriate that we should consider the more expansive and inclusive answer as well.
John 13:34-35 KJV
(34) A new commandment I give unto you, That ye love one another; as I have loved you, that ye also love one another.
(35) By this shall all men know that ye are my disciples, if ye have love one to another.
If the question is shifting to boundaries of doctrine, I would say that we would first need to look to the fruits of the spirit. As it is written, if we have not his spirit we are none of his (Romans 8:9).
1 John 3:23-24 KJV
(23) And this is his commandment, That we should believe on the name of his Son Jesus Christ, and love one another, as he gave us commandment.
(24) And he that keepeth his commandments dwelleth in him, and he in him. And hereby we know that he abideth in us, by the Spirit which he hath given us.
Matthew 7:15-17 KJV
(15) Beware of false prophets, which come to you in sheep's clothing, but inwardly they are ravening wolves.
(16) Ye shall know them by their fruits. Do men gather grapes of thorns, or figs of thistles?
(17) Even so every good tree bringeth forth good fruit; but a corrupt tree bringeth forth evil fruit.
We are told that the Spirit will guide us into all truth (John 16:13). That does not necessarily mean that everyone who has subjected themselves to that spirit is in perfect truth already. Even Apollos who was said to be "fervent in the spirit" had room to learn the way of God more perfectly (Acts 18:26). In our example here Apollos was not perfect in doctrine, but because he was fervent in the spirit, truth was able to further manifest as the fruit of that spirit (Ephesians 5:9).
James 5:19-20 KJV
(19) Brethren, if any of you do err from the truth, and one convert him;
(20) Let him know, that he which converteth the sinner from the error of his way shall save a soul from death, and shall hide a multitude of sins.
1 John 5:16-17 KJV
(16) If any man see his brother sin a sin which is not unto death, he shall ask, and he shall give him life for them that sin not unto death. There is a sin unto death: I do not say that he shall pray for it.
(17) All unrighteousness is sin: and there is a sin not unto death.
I am not sure that looking for boundaries is the right way to go about this... but where there the Spirit is we find unity in Christ and the law of love. Some doctrines may be trivial (or even foolish) and have no real influence on our faith, but admittedly other doctrines (such as a denial of the resurrection of the dead) may be more damaging even to the overthrowing of our faith.
2 Timothy 2:16-19 KJV
(16) But shun profane and vain babblings: for they will increase unto more ungodliness.
(17) And their word will eat as doth a canker: of whom is Hymenaeus and Philetus;
(18) Who concerning the truth have erred, saying that the resurrection is past already; and overthrow the faith of some.
(19) Nevertheless the foundation of God standeth sure, having this seal, The Lord knoweth them that are his. And, Let every one that nameth the name of Christ depart from iniquity.
So how does Paul address this problem? He names two people who have erred... and he addresses their specific error. Ultimately it is up to God who knows who are his and who are not... and that we who are his should depart from iniquity. That leads us back to James, "if any of you do err from the truth, and one convert him... [he] shall save a soul from death."
Ultimately those with the spirit of Christ will love their neighbors, love their brethren, and also love truth
that they might be saved (see also 2 Th 2:10). While some doctrines may naturally oppose the spirit of Christ or weaken the faith of Christ. I would count the spirit of Christ (rather than any specific doctrine) as being the defining aspect of those who are brothers in Christ.