Charles Ellicott Commentary Mark 1:34

"And he healed many that were sick with divers diseases, and cast out many demons; and he suffered not the demons to speak, because they knew him." — Mark 1:34 (ASV)

And suffered not the devils to speak.—St. Luke (Luke 4:41) gives the reason for the prohibition more distinctly. The demoniacs had cried out, Thou art the Son of God. They knew that He was the Christ.