The man is what his heart is. If this be dead to God, then nothing in him is alive. If this be right with God, all will be right. As the mainspring of a watch sets all its wheels and parts in motion, so as a man “thinketh in his heart, so is he” (Prov. 23:7). If the heart be right, the actions will be. As a man's heart is, such is his state now and will be hereafter: if it be regenerated and sanctified there will be a life of faith and holiness in this world, and everlasting life will be enjoyed in the world to come. Therefore, “Rather look to the cleansing of thine heart, than to the cleansing of thy well; rather look to the feeding of thine heart, than to the feeding of thy flock; rather look to the defending of thine heart, than to the defending of thine house; rather look to the keeping of thine heart, than to the keeping of thy money” (Peter Moffat, 1570).
“Keep thy heart with all diligence, for out of it are the issues of life” (Prov. 4:23). The “heart” is here put for our whole inner being, the “hidden man of the heart” (1 Peter 3:4). It is that which controls and gives character to all that we do. To “keep”—garrison or guard—the heart or soul is the great work which God has assigned us: the enablement is His, but the duty is ours. - A.W. Pink