True Christianity is defined in OP.
Unless you are saying all forms of Christianity is false.
I don't know the point of your OP. I've found that those who want to set boundaries are often trying to exclude people. There is a broad spectrum of Christian beliefs among those who say they believe in Christ. Mormons say this, but there are many who insist they're not real Christians. It reminds me of the gospels that record more than once the disciples trying to keep people from Jesus. Things aren't so different today.
Don't get me wrong, doctrines are important. But when we start saying that a certain quantity of error leads to damnation, then the frightening question is how much? Who decides how much and who can be absolutely certain they themselves don't have beliefs that will jeopardize their salvation?
This is why I don't doubt anyone who says they love the Lord and follow him. If their faith is sincere then no amount of error is going to hurt them. God isn't trying to send as many people to hell as possible, he's trying to save as many as possible.
Mormons aren't Christians. Muslims aren't Christians.
The boundary exists. There will be people that get excluded. Just because you fear it doesn't mean it doesn't exists. It is not us who excludes, but God. Therefore, why would you fear man's opinions?
You can have sincere faith in Satan as your Lord and definitely go to Hell. Sincere faith in truth saves. Sincere faith in falsehoods are fatal.
Luke 13
22 He went on his way through towns and villages, teaching and journeying toward Jerusalem. 23 And someone said to him, “Lord, will those who are saved be few?” And he said to them, 24 “Strive to enter through the narrow door. For many, I tell you, will seek to enter and will not be able. 25 When once the master of the house has risen and shut the door, and you begin to stand outside and to knock at the door, saying, ‘Lord, open to us,’ then he will answer you, ‘I do not know where you come from.’ 26 Then you will begin to say, ‘We ate and drank in your presence, and you taught in our streets.’ 27 But he will say, ‘I tell you, I do not know where you come from. Depart from me, all you workers of evil!’ 28 In that place there will be weeping and gnashing of teeth, when you see Abraham and Isaac and Jacob and all the prophets in the kingdom of God but you yourselves cast out. 29 And people will come from east and west, and from north and south, and recline at table in the kingdom of God. 30 And behold, some are last who will be first, and some are first who will be last.”
Matthew 7
13 “Enter by the narrow gate. For the gate is wide and the way is easy that leads to destruction, and those who enter by it are many. 14 For the gate is narrow and the way is hard that leads to life, and those who find it are few.
15 “Beware of false prophets, who come to you in sheep's clothing but inwardly are ravenous wolves. 16 You will recognize them by their fruits. Are grapes gathered from thornbushes, or figs from thistles? 17 So, every healthy tree bears good fruit, but the diseased tree bears bad fruit. 18 A healthy tree cannot bear bad fruit, nor can a diseased tree bear good fruit. 19 Every tree that does not bear good fruit is cut down and thrown into the fire. 20 Thus you will recognize them by their fruits.
21 “Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but the one who does the will of my Father who is in heaven. 22 On that day many will say to me, ‘Lord, Lord, did we not prophesy in your name, and cast out demons in your name, and do many mighty works in your name?’ 23 And then will I declare to them, ‘I never knew you; depart from me, you workers of lawlessness.’
