John Calvin Commentary Exodus 4:5

"That they may believe that Jehovah, the God of their fathers, the God of Abraham, the God of Isaac, and the God of Jacob, hath appeared unto thee." — Exodus 4:5 (ASV)

That they may believe. This spectacle, then, was not shown to Moses only once, but the power was also imparted to him to repeat the miracle frequently. This was both to gain credibility with the Israelites and to repress the audacity of Pharaoh. For although the sentence is incomplete, there is no ambiguity in the meaning: namely, that Moses was armed with power from heaven to make his vocation sure, so that no one would doubt him to be a Prophet divinely commissioned.

It would be tedious here to elaborate specifically on the use of miracles; it is sufficient to briefly state that they sometimes serve as preparations for faith and sometimes for its confirmation.

We see an example of both in the metamorphosis of the rod. By this, Moses was more animated and encouraged to gather strength, although he already believed God’s promise. The Israelites, however, who were both incredulous and unteachable, were by this means prepared and compelled to believe.

Besides, the miracle opened a door of faith for the Israelites, so that, being persuaded of his prophetic office, they might submit to being taught. Meanwhile, he himself was led on to greater assurance and perseverance.

For although the Almighty begins further back, referring to the adoption of the patriarchs (and this was calculated to lay the foundation of their hope of redemption), it still does not follow that they were prepared to receive Moses until the authority of his ministry had been established. Therefore, I have said, their faith was begun by the miracle.