How do you know, if one doesn't believe the "right" things about God, they're serving the right god?
And because the Holy Spirit leads us into ALL truth (granted for some that is a lengthier process than for others), if we do not have ALL truth, is there enough error that we may not truly be saved, regardless of our heart?
Because my bible says:
Jer. 17:9 The heart is more deceitful than all else
And is desperately sick;
Who can understand it?
10 “I, the Lord, search the heart,
I test the mind,
Even to give to each man according to his ways,
According to the results of his deeds.
Then there's Job 5:
7 For man is born for trouble,
As sparks fly upward.
And this:
Luke 6:45 The good man out of the good treasure of his heart brings forth what is good; and the evil man out of the evil treasure brings forth what is evil; for his mouth speaks from that which fills his heart.
Matt. 12:34 Brood of vipers! How can you, being evil, speak good things?
For out of the abundance of the heart the mouth speaks.
The heart, as the seat of the will, in a right view of God and His things, exactly determines eternal life or eternal death. 99% of truth mixed with 1% of error is still error.
That's why we need to read scripture, ask questions of God, pray for wisdom (if you lack ask God Who gives to all liberally and without reproach) and attend/assemble under godly preaching. And if one is sincere (more self-delusion, I'm afraid) would not God, our Father, give good things to those who ask Him?
Absolutely!
Scriptute doesn't bid us "guard our hearts" for nothing.