'Heart' in the Bible

Since by your obedience to the truth you have purified yourselves for a sincere love of the believers, [see that you] love one another from the heart [always unselfishly seeking the best for one another],

but let it be [the inner beauty of] the hidden person of the heart, with the imperishable quality and unfading charm of a gentle and peaceful spirit, [one that is calm and self-controlled, not overanxious, but serene and spiritually mature] which is very precious in the sight of God.