Christians May Lack Assurance for a Time - Why. Larger Cat. Q 81

It is one thing to fall from the truth of grace; another thing to lose the comfortable sense of it. The joy of faith may be suspended, when the acts and habits of faith remain firm and unshaken. As the brightest morning may be followed with clouds and tempests; so our clearest discoveries of our interest in the love of God may be followed with the withdrawment of the light of his countenance, and we be left under many discouraging circumstances concerning our state, having lost the assurance we once had. (Exposition of the Larger Catechism, Q. 81)