Albert Barnes Commentary Psalms 91:16

"With long life will I satisfy him, And show him my salvation." — Psalms 91:16 (ASV)

With long life will I satisfy him - The margin here is "length of days"; that is, days lengthened out or multiplied. The meaning is, I will give him length of days as he desires, or until he is satisfied with life, implying:

1. that it is natural to desire long life;
2. that long life is to be regarded as a blessing (compare Proverbs 3:2, Proverbs 3:16; Exodus 20:12);
3. that the tendency of religion is to lengthen out life, since virtue, temperance, regular industry, calmness of mind, moderation in all things, and freedom from excesses in eating and drinking—to all of which religion prompts—contribute to health and to length of days (Psalms 37:9; Psalms 55:23); and
4. that a time will come, even under this promised blessing of length of days, when a man will be "satisfied" with living; when he will have no strong desire to live longer; when, under the infirmities of advanced years, and under his lonely feelings from the fact that his early friends have fallen, and under the influence of a bright hope of heaven, he will feel that he has had enough of life here, and that it is better to depart to another world.

And show him my salvation - In another life, after he shall be "satisfied" with this life. The promise extends beyond the grave: Godliness is profitable unto all things, having promise of the life that now is, and of that which is to come. See the notes at 1 Timothy 4:8. Thus, religion blesses man in this life, and blesses him forever. In possession of this, it is a great thing for him to live long; and then it is a great thing to die—to go to be forever with God.