John Calvin Commentary Isaiah 31:4

"For thus saith Jehovah unto me, As the lion and the young lion growling over his prey, if a multitude of shepherds be called forth against him, will not be dismayed at their voice, nor abase himself for the noise of them: so will Jehovah of hosts come down to fight upon mount Zion, and upon the hill thereof." — Isaiah 31:4 (ASV)

For so has Jehovah said to me. The Prophet adds this verse so that it may not be thought that the Lord leaves us lacking necessary means; for if, while he forbids us to place our confidence in creatures, he did not promise us any assistance, we might complain that he gave reason for despair and not for consolation; as we saw previously, that people are more careful and attentive than they ought to be, because they think that they will be lacking in foresight if they rest satisfied with God alone and abstain from forbidden means.

He therefore takes away every excuse when he promises that he will be a faithful guardian to us; for what pretext can be left if we despise the salvation which he offers to us freely? It is therefore as if he had said, “The Lord assists, and will assist; he forbids you to ask assistance from the Egyptians.” By comparing himself to a lion, a very powerful animal and intensely focused on prey, he employs a very appropriate comparison to show that he is to the highest degree both able and willing to defend us.

In the second part of the comparison, the Prophet elaborates extensively on the great eagerness with which the Lord takes hold of his people, keeps them near himself, preserves them from being carried off, and defends them against all dangers; while he also points out that strength and power which no arms and no forces can resist. Now, it is impossible that comparisons should apply in every detail, nor is that necessary, but they ought to be suitable to the subject being discussed. Since therefore we know that the Lord loves us so much and takes such care of us, must we not be worse than mad if we despise him, and seek other aids, which will not only be useless but destructive to us?