Deuteronomy 32:47

For it [is] not a vain thing for you
That is, the law is not, and the observance of it in them and their children; it was no light and trifling matter, but of great importance and consequence, obedience to it being attended with rewards, and disobedience with punishment:

because it [is] your life:
if obeyed, the means of a comfortable and happy life, in the enjoyment of all good things, of the preservation and continuance of it to a length of time; and long life was always reckoned a great temporal mercy:

and through this thing ye shall prolong [your] days in the land
whither you go over Jordan to possess it;
that is, through their obedience to the law they should live long in the land of Canaan, which they were just going to possess; which explains what is meant by its being their life; their comfortable living and continuance in the land depended on their obedience to the law; see ( Isaiah 1:19 Isaiah 1:20 ) .