John Gill Commentary Matthew 12:28

"But if I by the Spirit of God cast out demons, then is the kingdom of God come upon you." — Matthew 12:28 (ASV)

But if I cast out devils by the Spirit of God
As it was certain he did, from the nature, use, and design of such miracles; and it could not be reasonably thought, that Satan would assist in what was so very opposite to his kingdom and interest, and was so serviceable to the cause and glory of Christ. All the three persons had a hand in the miracles of the Messiah; they were done by Christ, in his Father's name, and by the power of the Spirit of God; from which the following inference may be justly deduced,

then the kingdom of God is come unto you :
meaning, either the Messiah himself; or rather, his kingdom, the Gospel dispensation, which both Christ and John had declared to be at hand; of which the performing of miracles, particularly the casting out of devils, whereby the kingdom of Satan was so much weakened, was a clear proof.