Wherefore I give you to understand, that no man speaking by the Spirit of God calleth Jesus accursed: and that no man can say that Jesus is the Lord, but by the Holy Ghost. No man speaking by the Spirit of God - It was granted on all hands that there could be no religion without Divine inspiration, because God alone, could make his will known to men: hence heathenism pretended to this inspiration; Judaism had it in the law and the prophets; and it was the very essence of the Christian religion. The heathen priests and priestesses pretended to receive, by inspiration from their god, the answers which they gave to their votaries. And as far as the people believed their pretensions, so far they were led by their teaching.
Both Judaism and heathenism were full of expectations of a future teacher and deliverer; and to this person, especially among the Jews, the Spirit in all the prophets gave witness. This was the Anointed One, the Messiah who was manifested in the person of Jesus of Nazareth; and him the Jews rejected, though he proved his Divine mission both by his doctrines and his miracles. But as he did not come as they fancied he would - as a mighty secular conqueror, they not only rejected but blasphemed him; and persons among them professing to be spiritual men, and under the influence of the Spirit of God, did so. But as the Holy Spirit, through all the law and the prophets gave Testimony to the Messiah, and as Jesus proved himself to be the Christ both by his miracles and doctrines, no man under the inspiration of the Divine Spirit could say to him anathema - thou art a deceiver, and a person worthy of death, etc., as the Jews did: therefore the Jews were no longer under the inspiration of the Spirit of God. This appears to be the meaning of the apostle in this place. No man speaking by the Spirit, etc.
And that no man can say that Jesus is the Lord - Nor can we demonstrate this person to be the Messiah and the Savior of men, but by the Holy Ghost, enabling us to speak with divers tongues, to work miracles; he attesting the truth of our doctrines to them that hear, by enlightening their minds, changing their hearts, and filling them with the peace and love of God.