Deuteronomy 31:6

Be strong and of a good courage ,.... The Septuagint version is,"play the men, and be strong;'be strong in the Lord, and in the power of his might, trusting and relying on him that goes before you; and so take heart, and be of good courage, and act the manly part; the apostle seems to refer to this passage, 1 Corinthians 16:13 ,

fear not, nor be afraid of them ; their enemies, though so numerous, so mighty, and some of them of a gigantic stature, and their cities strong and well fenced:

for the Lord thy God, he it is that doth go with thee : in comparison of whom, numbers of men, their strength of body, and fortified places, signify nothing:

he will not fail thee, nor forsake thee ; not fail to fulfil his promises to them, not leave them till he had given them complete victory over their enemies, put them into the possession of their land, and settled them in it. This promise, though made to literal Israel, belongs to the spiritual Israel of God, and is made good to every true believer in the Lord; see Hebrews 13:5 .