John Calvin Commentary Isaiah 33:21

"But there Jehovah will be with us in majesty, a place of broad rivers and streams, wherein shall go no galley with oars, neither shall gallant ship pass thereby." — Isaiah 33:21 (ASV)

Because there the mighty Jehovah will be to us. The two particles כי ים (ki im) often serve as a double affirmative, but here a reason is given, and they could even be appropriately rendered, For if; but I willingly retain what is clearer. The Prophet gives the reason why the Church, which appears to resemble a movable tent, surpasses in stability the best-founded cities. It is because the Lord is in the midst of her, as it is also said (Psalms 46:5), and therefore she shall not be moved. If we separate the Church from God, it will immediately fall without any attack, for it will consist of humans only, than whom nothing is weaker or more frail.

Will be to us a place of rivers. When God dwells with us, he confirms and supports what is naturally feeble. He serves us as a very strong fortress, a very broad ditch, and walls and rivers surrounding the city on every side. He alludes to the situation of the city Jerusalem, which had only a small rivulet, and not large and rapid rivers, like those of Babylon and other cities; for in another passage (Isaiah 8:6) he urged them to rest satisfied with the power of God alone, and not to covet those broad rivers. As if he had said, “Our strength shall be invincible, if God rules over us; for under his guidance and direction we shall be abundantly fortified.”

There shall not pass a ship with oars. Large rivers can have this disadvantage: they can give access to enemies, allowing them to approach with their ships closer than desired. Thus, very frequently, what appeared to be a benefit is found to be harmful. But while the Lord says that he will be a river, he also says that there will be no reason to dread such a disadvantage, and that enemies will not be allowed to approach. He mentions two kinds of ships, long ships and ships of burden, to show that enemies will be shut out in every possible way. From this we should draw a very useful doctrine: that the hope of safety should be sought from no other than God alone, and that it is futile to gather various means of defense, which will be useless, and even harmful, if he is not on our side.