John Calvin Commentary Joshua 1:5

"There shall not any man be able to stand before thee all the days of thy life. as I was with Moses, so I will be with thee; I will not fail thee, nor forsake thee." — Joshua 1:5 (ASV)

There shall not any man, etc. As a contest was about to be waged with numerous and warlike enemies, it was necessary thus to inspire Joshua with special confidence. Without this, the promise of delivering the land which God had given would repeatedly have become obscured, for how vast was the undertaking to overthrow so many nations! This objection therefore is removed. And to better free him from all doubt, he is reminded of the victories of Moses, by which God had made it evident that nothing was easier for him than utterly to defeat any army, however great and powerful. Joshua, therefore, is instructed to see in the assistance given to Moses the future outcome of the wars which he was to undertake under the same guidance and protection. For the series of favors is continued without interruption to the successor.

What follows serves the same purpose, though it is more fully expressed in the words, I will not fail you, etc. Therefore, the Apostle (Hebrews 13:5), wishing to steer believers away from greed, applies these words to calm all anxieties and suppress all excessive fears. Indeed, the distrust that arises from anxiety ignites such tumultuous feelings in us that at the slightest sign of danger, we become agitated and miserably torment ourselves until we feel assured that God will both be with us and be more than sufficient for our protection. And indeed, while he prescribes no other cure for our timidity, he reminds us that we should be satisfied with his present help.