Caravaggio, Saint Jerome Writing (c. 1605), Galleria Borghese. Public domain.
4 verses in the King James Version. Word study for “heart” →
Having the understanding darkened, being alienated from the life of God through the ignorance that is in them, because of the blindness of their heart:
Speaking to yourselves in psalms and hymns and spiritual songs, singing and making melody in your heart to the Lord;
Servants, be obedient to them that are your masters according to the flesh, with fear and trembling, in singleness of your heart, as unto Christ;
Not with eyeservice, as menpleasers; but as the servants of Christ, doing the will of God from the heart;