Form of Sound Words, p. 634-636
“In the first place, every deviation from truth is a sin. It is not simply a sin for me to do a wrong act, but it is a sin for me to believe a wrong doctrine. Lately our ministers have absolved us all from obeying God in our judgments; they have told us point blank, (many of them, in their drawing-rooms, and some of them in the pulpit) that we shall never be asked in the day of judgment what we believed. We have been told that for our actions we shall be responsible, but for our faith we shall be irresponsible (or something very much like it). They have told us plainly, that the God who made us, although He has authority over our hands, our feet, our eyes, and our lips, hath but little authroity over our judgment; they have told us, that if we make ver such bluders in divinity, they are no sins, so long as we can live right lives! But is that true? No: the whole man is bound to serve God; and if God gives me a judgment (i.e.: the ability to make discernments), I am bound employ that judgmentent in His service; and if that judgment receive untruthrth, it has received stolen goods, and I have sinned as much as if I put forth my hand to take my neighbor's goods. There may be degrees in in the sin. If it be a sin of ignorance, it is nevertheless a sin; but it is not so heinous as a sin of negligence, which I fear it is with many. I tell you, beloved, if for instance, baptism be not immersion, I commit a sin ever time I practice it; and if it be, my brother commits a sin who does not practice it. If election be true, I am committing a sin if I do not believe it; and if final perseverance be true, I am committing a sin before Almighty God, if I do not receive it; and if it be not true, then I sin in embracing what is not Scriptural! Error in doctrine is as much a sin as error in practice! In everything we are bound to serve our God with all our might, exercising those powers of judging and believing which He has given to us. And I warn you, Christians, not to thing it is a little thing to hold faith with a feeble hand: it is a sin every time you do anything which makes you waver in the faith of Jesus Christ.
Remember, too, that error in doctrine is not only a sin, but a sin which has a great tendency to increase. When a man once in his life believes a wrong thing, it is marvelous how quickly he believes another wrong thing. Once you open the door to a false doctrine – (Satan says it is but a little one) – yes, but he only puts the little one in like the small end of the wedge, and he means to drive in a larger one; and he will say it is only a little more, and a little more, and a little more. The most dammable heretics who ever perverted the faith of God erred by littles and littles; those who have gone the widest from truth have only gone so slowly … by degrees...
Take care, Christians, if you commit one error, you cannot tell how many more you will commit. “Hole fast the form of sound words,” (that is what the apostle Paul commanded Timothy; because error in doctrine inevitably leads to error in practice. When a man believers wrongly, he will soon act wrongly. Faith has a great influence on our conduct. As a man's faith, so he is. If you begin to imbibe erroneous doctrines, they soon have an effect on your practice.) Keep fast to the bulwarks of your Fathers' faith. If you do not, the enemy will make sad havoc with you. “Hold fast (as Scripture declares) the form of sound words which was delivered unto you.”