Matthew 17:12

But I say unto you
A way of speaking used by Christ, when he opposes and contradicts any of the tenets of the Scribes and Pharisees; see ( Matthew 5:22 Matthew 5:28 Matthew 5:32 Matthew 5:34 Matthew 5:39 Matthew 5:44 ) "that Elias is come already"; the person that was signified by, and prophesied of, under the name of Elias: for Christ refers not to the late appearance of Elias on the mount, but to the coming of a certain person some time ago; who came in the power and spirit of Elias, and was the forerunner and harbinger of him, the Messiah; as was said of him he should, "and they knew him not"; that is, the Scribes and Pharisees, who believed that Elias would come before the Messiah; and yet when he who was designed by him was come, they knew him not, they did not know him to be the Elias; they knew him under the name of John the Baptist, and seemed pleased with his ministry for a while, but afterwards rejected his doctrine and baptism, which is referred to in the next clause:

but have done unto him whatsoever they listed;
they did not believe what he said, nor repent upon his preaching to them; they rejected the counsel of God he declared, not being baptized of him; they treated him with indignity and contempt, charging him with having a devil, and were well pleased when Herod put him to death; some of whom were doubtless among those that sat at meat with him; for whose sake, as well as for his oath's sake, he ordered the execrable murder to be committed:

likewise also shall the son of man suffer of them.
Christ takes this opportunity to confirm what he had said in the preceding chapter, concerning his sufferings and death; and his meaning is, that as sure as John the Baptist had suffered indignities, and death itself, so sure should the son of man suffer like things; if not from the same individual persons, yet from that generation of men.