The devil doesn't sleep

I just opened at random my copy of Thomas à Kempis' The Imitation of Christ (c. 1420) and immediately came across this:

I have found no person so devout that he or she never suffered sometimes a withdrawing of grace, or a decrease in zeal.

Never did a saint exist so lost in God, or so illuminated, who sooner or later experienced no temptation.

Reason? No person is worthy to meditate about God who has not first struggled with trials for God's sake.

In fact, temptation now means peace later. For those who pass the tests, heavenly comfort follows that's a promise. Here is the promise:

"To overcomers," God says, "I give food from the tree of life."

Moreover, divine consolation comes to fortify us against adversities; temptation comes also to keep us from pride over any good we do.

The devil doesn't sleep; the flesh hasn't died yet; therefore, always stay fit for battle: for on both your right and left your enemies never rest.

Thomas à Kempis, The Imitation of Christ