Secret sins are in some respects more dangerous than open sins.  The more inward and secret the disease is, the more the man is in danger of losing his life.  There are no fevers so dangerous as those that prey upon the spirits and inward parts:  so there are no sins so pernicious to the souls of men as those that are most inward and secret.  Secret sins often reign in the souls of men most powerfully when they are least apparent.

From Smooth Stones (Spurgeon’s collected sayings of Thomas Brooks)