The first and great work of a Christian is about his heart. There it is that God dwells by his Spirit, in his saints; and there it is that sin and Satan reign, in the ungodly. The great duties and the great sins are those of the heart. There is the root of good and evil: the tongue and life are but the fruits and expressions of that which dwells within.
Adapted from The Practical Works of the Rev. Richard Baxter, 2:531.