We might have reckoned that there would be ungracious men mingled with the people of God, since it is clear to every thoughtful man that this must be one of the craftiest designs of Satan. In what way can Satan so seriously damage the church of God as by thrusting unworthy persons into it? While men slept, the enemy came and sowed tares among the wheat, because the tares would take away the nourishment from the wheat and help to choke it, and prevent it from yielding so rich a harvest…Satan knows right well that one devil in the church can do far more than a thousand devils outside her bounds. He understands that all the blasphemers, and atheists, and free-thinkers, and so on that ever assailed the bastions of the church of God could not do one tenth as much mischief to her as those who pretend to be followers of the bleeding Lamb, and in secret are crucifying the Lord afresh, and putting him to an open shame.1
Charles H. Spurgeon
12 These are hidden reefs at your love feasts, as they feast with you without fear, shepherds feeding themselves; waterless clouds, swept along by winds; fruitless trees in late autumn, twice dead, uprooted;