What is commonly called a vow (that is, pledging oneself without or with penalties if it is broken) is, in itself, the fruit of self-confidence and energy of the flesh—two things which mark fallen men, and which God abhors.
Conscience is a natural thing, and came in with the fall in Eden; for till then, all in man was right, and he could not think God had anything against His own unmarred handiwork—which man was. I notice conscience here because with it comes the question of honesty and uprightness which are of great moment to the Christian. But if conscience is the knowledge which man has before God, as to God's thoughts of this or of that, the unconverted man has no light of revelation in his soul; and the light which comes in at conversion makes everything manifest. A conscience must be placed in the light and have the teaching of God ere it can rest satisfied that it knows what is right.
Many men have vowed to commit a sin, and used the vow as the excuse for doing it; and yet, had anyone said to them, Dare you say to God. "Thou wouldst that I should commit this murder," or whatever the sin be, they would reply, "Certainly I cannot. Even nature tells me it is a sin." If a man vowed to be an apostle, or to convert many people, or not to marry, etc., let him confess his sin, and leave himself in God's hand. He has assumed power to be in himself, and it is not there.