Charles H. Spurgeon on Psalms 119:38

Stablish thy word unto thy servant. Make me sure of thy sure word: make it sure to me and make me sure of it. If we possess the spirit of service, and yet are troubled with sceptical thoughts we cannot do better than pray to be established in the truth. Times will arise when every doctrine and promise seems to be shaken, and our mind gets no rest: then we must appeal to God for establishment in the faith, for he would have all his servants to be well instructed and confirmed in his word. But we must mind that we are the Lord's servants, for else we shall not long be sound in his truth. Practical holiness is a great help towards doctrinal certainty: if we are God's servants he will confirm his word in our experience. "If any man will do his will, he shall know of the doctrine"; and so know it as to be fully assured of it. Atheism in the heart is a horrible plague to a God fearing man, it brings more torment with it than can well be described; and nothing but a visitation of grace can settle the soul after it has been violently assailed thereby. Vanity or falsehood is bad for the eyes, but it is even worse when it defiles the understanding and casts a doubt upon the word of the living God.

Who is devoted to thy fear, or simply "to thy fear." That is, make good thy word to godly fear: wherever it exists; strengthen the whole body of reverent men. Stablish thy word, not only to me, but to all the godly ones under the sun. Or, again, it may mean "Stablish thy word to thy fear, "namely, that men may be led to fear thee; since a sure faith in the divine promise is the fountain and foundation of godly fear. Men will never worship a God in whom they do not believe. More faith will lead to more godly fear. We cannot look for the fulfilment of promises in our experience unless we live under the influence of the fear of the Lord: establishment in grace is the result of holy watchfulness and prayerful energy. We shall never be rooted and grounded in our belief unless we daily practise what we profess to believe. Full assurance is the reward of obedience. Answers to prayer are given to those whose hearts answer to the Lord's command. If we are devoted to God's fear we shall be delivered from all other fear. He has no fear as to the truth of the word who is filled with fear of the Author of the word. Scepticism is both the parent and the child of impiety; but strong faith both begets piety and is begotten of it. We commend this whole verse to any devout man whose tendency is to scepticism: it will be an admirable prayer for use in seasons of unusually strong misgivings.