Charles H. Spurgeon on Psalms 121:3

He will not suffer thy foot to be moved. Though the paths of life are dangerous and difficult, yet we shall stand fast, for Jehovah will not permit our feet to slide; and if he will not suffer it we shall not suffer it. If our foot will be thus kept we may be sure that our head and heart will be preserved also. In the original the words express a wish or prayer, —"May he not suffer thy foot to be moved." Promised preservation should be the subject of perpetual prayer; and we may pray believing; for those who have God for their keeper shall be safe from all the perils of the way. Among the hills and ravines of Palestine the literal keeping of tim feet is a great mercy; but in the slippery ways of a tried and afflicted life, the boon of upholding is of priceless value, for a single false step might cause us a fall fraught with awful danger. To stand erect and pursue the even tenor of our way is a blessing which only God can give, which is worthy of the divine hand, and worthy also of perennial gratitude. Our feet shall move in progress, but they shall not be moved to their overthrow. He that keepeth thee will not slumber, —or "thy keeper shall not slumber". We should not stand a moment if our keeper were to sleep; we need him by day and by night; not a single step can be safely taken except under his guardian eye. This is a choice stanza in a pilgrim song. God is the convoy and body guard of his saints. When dangers are awake around us we are safe, for our Preserver is awake also, and will not permit us to be taken unawares. No fatigue or exhaustion can cast our God into sleep; his watchful eyes are never closed.