Self deception

Men profess they know the truth; but they know it not in its proper order, in its harmony and use.  It leads them not to Christ, it brings not Christ unto them; and so is lifeless and useless.  Hence, ofttimes, none are more estranged from the life of God than such as have much notional knowledge of the doctrines of the Scripture.  For they are all of them useless, and subject to be abused, if they are not improved to form Christ in the soul, and transform the whole person into his likeness and image.

John Owen