John Calvin Commentary Acts 7:31

"And when Moses saw it, he wondered at the sight: and as he drew near to behold, there came a voice of the Lord," — Acts 7:31 (ASV)

He wondered at the vision. Let us understand that God used to deal with our ancestors in this way, so that they might surely know His majesty; for He intended to make a clear distinction between the visions He showed and the deceptive tricks of Satan. And this certainty is all the more necessary, for what credibility would the oracles of God otherwise have, in which the covenant of eternal life is contained?

Therefore, since the true foundation of faith is this alone—to have God as its author—He must necessarily and undoubtedly declare that it is He who speaks. Again, since Satan walks about continually, and by many strange tricks inserts himself, and has so many ways to deceive—and especially since he deceptively uses the name of God—we must be very careful of his deceptions.

We see how in past times he deceived all nations, and how he deceives the Papists also. For all the monstrous superstitions, all the delusions of error which existed in past times and still prevail in Popery, proceeded from dreams, visions, and false revelations; indeed, furthermore, even the Anabaptists derive their illusions from the same source.

Therefore, the only remedy is this: that God Himself distinguish by certain marks those visions which He shows. For then we are without danger of erring, when He has revealed His majesty to us. For this reason, the mind of Moses was struck with awe, and then afterwards he draws near to consider. After he has come nearer, the Lord touches him with a more vivid sense of His presence (I indeed confess this), so that he is afraid.

For I confess that there is none of these things which Satan cannot imitate, though he does so falsely, like an ape. And the Lord does not only show Himself by such signs, but, helping our slowness to understand, He also opens our eyes so that we may not be deceived. Again, the Holy Ghost impresses on our minds certain marks and tokens of God’s presence, so that no doubt may remain.