heart

The perfect Law of God

The law of the Lord is perfect, restoring the soul; The testimony of the Lord is sure, making wise the simple.

The precepts of the Lord are right, rejoicing the heart; The commandment of the Lord is pure, enlightening the eyes.

Psalms 19:7-8 (NASB)

Salvation through faith

That if you confess with your mouth Jesus as Lord, and believe in your heart that God raised Him from the dead, you will be saved;

for with the heart a person believes, resulting in righteousness, and with the mouth he confesses, resulting in salvation.

Romans 10:9-10 (NASB)

Do not walk in darkness

So this I say, and affirm together with the Lord, that you walk no longer just as the Gentiles also walk, in the futility of their mind,

being darkened in their understanding, excluded from the life of God because of the ignorance that is in them, because of the hardness of their heart;

Ephesians 4:17-18 (NASB)