Who can understand his errors? cleanse thou me from secret faults. Psalm 19:12
…While we insist upon it that not every error is fatal, we wish to guard against the misconception, or false conclusion, according to which it is forthwith pronounced indifferent.
Fatal or indifferent are not the only alternatives. No sin in practice is indifferent, yet our hopes were indeed vain, if all were absolutely fatal.
Every sin in doctrine is dangerous, whether fundamental or not. It is so, not only because we are accountable for the light which we might have enjoyed, and the acceptance of which might have preserved us from the mistakes made, but also because one error opens the way for another and a more dangerous. Each false doctrine is a step, however small, towards the establishment of a system growing out of our own minds, and subversive of that which is revealed. It is therefore of most grave importance, even though by the restraining grace of God it does not always eventuate in such ruin. The consequences may be averted, and cannot, therefore, fairly be considered as necessarily involved in the error, and the sin may be forgiven among those secret faults, for the pardon of which believers daily pray. — Ps. 19:12.
From: Loy, Matthias. The Unity of the Church. 1857. LutheranLibrary.org