But I say unto you ... - Christ closes this address to his malignant and wicked hearers by a solemn declaration that for these things God would bring them into judgment. Therefore. They who had spoken so malignantly against him, could not escape.
Idle word - This literally means a vain, thoughtless, useless word; a word that accomplishes no good. Here it means, evidently, “wicked, injurious, false, malicious, for such” were the words which they had spoken.