Take heed, brothers, lest there be in any of you an evil heart of unbelief, in departing from the living God.
Anybody who has common power of observation, must be struck by the wonderful things which are constantly attributed in Holy Writ to faith, or believing in the word of God — be it what it will — and especially in that revelation of Himself which He has made in Christ Jesus (see Romans 4:5; Galatians 3:11; Mark 9:23; Matthew 17:20; Luke 8:48, &c.). But a second thing, equally beyond doubt, is universally asserted of this Divine grace: that from a true faith springs of necessity, like a tree from its root, a corresponding obedience, a bringing forth the fruits of the Spirit. To be a believer and a doer of the Word are the very same thing. Faith or belief is holy living, and holy living is faith, being one and indivisible; so that the inward principle, denoted by the term faith, comprehends all things which, whether in our justification or sanctification, are made by the word of God essential to our everlasting salvation. Now, then, this nature of ours, which makes us what we are — men, and not angels or brutes — is not a single or a simple thing, but is made up of at least two parts, what we call our heart and our head, or our understanding. The first, that by which we feel, and love, and hate, and have a choice or will; and the other, that by which we see what is right and true, and in a lower form of it, reason about the things of the world in which we live, and which our senses present to us. Some things belong only to the head, and if that consents to them, it is enough; it is the belief which belongs to that kind of truths. Such are many things in numbers, and what is called science, and many matters of fact; men and people, for instance, mentioned in books, and many concerns of this life; the heart or will has nothing to do with them one way or other. But other things have not only a true and a false, but a right and wrong about them, and when admitted as true, make it absolutely necessary for us to approve them and to act upon them, and by reason of them; and since, therefore, they touch at once the heart and the head, they cannot be really believed, unless those two parts of our nature go together. When they do so, then, and then only can we, indeed, and in truth, be said to believe them. And when anything is thus admitted, and beats down all opposition before it, and occupies all our nature, all the spiritual being, whatever of it by which we think and feel, is made to act as God intended it to do. As a wheel rolls when the needful force pushes it in a particular direction, or any other machine moves when the spring is touched, so does the man. He is agitated, he is moved; thought and feeling go forth into visible actions he does and acts accordingly; his nature is at unity with itself, and all obstacles being overpowered, impels him in one way. Now, the solemn thing for us to consider is this, that such is the case with all that God has revealed to us in the glorious gospel of His Son. It is not made up of things to be received into the head, only as part of us, and to be kept like book knowledge, outside of the soul, but it is to be accepted by our whole and entire soul. You see, then, in an instant, what a number of powerful enemies there are within us, to divide, even in things of themselves most clear, the heart and will from the head, and to prevent that living and true belief in Christ, and in His gospel without which no soul of man can be saved. What a fearful alienation from God, as a spiritual God, there is in the heart, whatever natural graces may adorn it! What an iron stubbornness of will and resolution to conform all things to itself, and not itself to the eternal law I Yet God, if He is God, is not a word, or a fancy, but an awful King, who must in all things be obeyed. Flowing from the same evil source, what an unspeakable repugnance there is to such a love of Christ, as shall have power over us. What vanities, what idolatries, what coldnesses! What an evil ally in the world about us, and the enemies — not of flesh and blood, but princedoms, dominations, and powers, even all the hosts of Satan — who rest not day or night, but toil to harden up the evil heart within us, to the destruction of all living faith, and the ruin of the soul.
Parallel VersesKJV: Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God.