When I do a discernment essay, I am often chastised and rebuked with an old chestnut of a comment that's becoming practically standard for those without discernment to post:
He alone is sovereign and fully knows all hearts!
God is sovereign. God knows the heart. But we do too.
If a teacher's doctrine has been proven false by comparing it with the Bible, then we DO know their heart! The Bible tells us this. Only God knows the hearts of the people, but if their teaching is not of the Lord, then the God who sees hearts has exposed those hearts to us by the verses of His word!
Their hearts are full of deceit. Colossians 2:8
Their hearts are filled with their own appetites. Romans 16:17-18
Their hearts are disguised with light. 2 Corinthians 11:13-15
Their hearts are full of greed. 2 Peter 2:3
Their hearts are ravenous. Matthew 7:15.
Their hearts are inwardly full of sensuality. Jude 1:4
Their hearts are full of secrets, such as destructive heresies. 2 Peter 2:1
Their hearts are full of intent to exploit. 2 Peter 2:3
Their hearts are full of fleshly passions. 2 Timothy 4:3
Their hearts are puffed up with conceit. 1 Timothy 6:4
His heart understands nothing. 1 Timothy 6:4
Their hearts are cunning and crafty. Ephesians 4:14
Their hearts serve the creature. Romans 1:25
Their hearts are slaves of corruption. 2 Peter 2:19.
Their hearts deny the Master who bought them. 2 Peter 2:1
Their hearts prophesy lies. Jeremiah 23:26
So whenever I expose a testimony as false or a teaching as false, or a teacher as false, using biblical proof, STOP saying that this is a bad activity because "only God knows the heart". He does, that's true, but He has shown us the heart of the false teacher in His word. He taught us this in His word for the purpose of being mature and discerning, and so we can learn for ourselves and also teach the younger to be edified and strong.