'Heart' in the Bible

Let him make his request in faith, doubting nothing; for he who has doubt in his heart is like the waves of the sea, which are troubled by the driving of the wind.

If any man among you seem to be religious, and bridleth not his tongue, but deceiveth his own heart, this man's religion is vain.

But if you have bitter envy and selfish ambition in your heart, don’t brag and deny the truth.

Is anyone among you suffering? He must pray. Is anyone joyful? He is to sing praises [to God].

Therefore, confess your sins to one another [your false steps, your offenses], and pray for one another, that you may be healed and restored. The heartfelt and persistent prayer of a righteous man (believer) can accomplish much [when put into action and made effective by God—it is dynamic and can have tremendous power].