The seat of sin, the Scripture everywhere assigns it to be the heart. There indwelling sin keeps its especial residence. It hath invaded and possessed the throne of God himself: “Madness is in the heart of men while they live” (Ecc 9:3). This is their madness, or the root of all that madness which appears in their lives. “Out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies” (Matt 15:19).
There are many outward temptations and provocations that befall men, which excite and stir them up unto these evils; but they do but as it were open the vessel, and let out what is laid up and stored in it. The root, rise, and spring of all these things is in the heart. Temptations and occasions put nothing into a man, but only draw out what was in him before.
Adapted from The Works of John Owen, 6:169.