Mark 3:11

And the unclean spirits, whensoever they beheld him, fell down before him, and cried, saying, Thou art the Son of God. And he charged them much that they should not make him known.

Unclean spirits ... For discussion of demon possession, see under Mark 1:24.

Thou art the Son of God ... This demonic witness was true, although proffered in keeping with some ulterior design of the evil one; and it is of the utmost significance that Jesus rejected this witness of the unclean. Two clear reasons for this rejection are: (1) it was premature for Jesus to be hailed as "the Son of God," a title with strong secular implications in the Hebrew mind and actually being equated with "King of Israel" (John 1:49). Had he permitted this title of himself to stand, Christ would have been hauled before the Romans for sedition. (2) If demons had been freely permitted to bear such testimony, it might have appeared to reinforce the slander of the Pharisees that he cast out demons by the prince of the demons (Mark 3:22).

Son of God ... must be understood here in its unique Messianic import. Any other meaning would not have served the demonic purpose. It should be noted that Christ did not deny their testimony as true, but on the other hand he forbade them to utter it.