Mainstream Christians regard anything that strays too far from the conventional Christian path as error, sin, or a combination of both.
They are suspicious of motivations to explore beyond well-established Christian boundaries and tend to consider such motivations to be "prideful".
As far as they are concerned, everything that can be known about Christianity has already been revealed. Thinking otherwise amounts to nothing more than arrogance and conceit, which can only be countered by a healthy dose of spiritual humility.
For example, challenge the traditional belief in the omnipotence of God and mainstream Christians will respond that such ideas do not stem from any potential deficiency in God, but rather from the very obvious deficiencies of man; from some errant, prideful humanistic attitude that erroneously dares to exalt man at the expense of God.
Pressed further, mainstream Christians will eventually assert that a non-omnipotent Christian God would repudiate the core essence of Christianity and make God un-worshipable.
Thus, an omnipotent God is the only sort of God mainstream Christians deem worthy of their human veneration.
But that's not an example of spiritual pride. No way.
Note added: I have no problem with Christians who believe in the omnipotence of God. Similarly, I have no problem with Christians who believe God may not be omnipotent. I believe a person can be a good, serious Christian either way.