Charles H. Spurgeon on Psalms 119:42

So shall I have wherewith to answer him that reproacheth me. This is an unanswerable answer. When God, by granting us salvation, gives to our prayers an answer of peace, we are ready at once to answer the objections of the infidel, the quibbles of the sceptical, and the sneers of the contemptuous. It is most desirable that revilers should be answered, and hence we may expect the Lord to save his people in order that a weapon may be put into their hands with which to rout his adversaries. When those who reproach us are also reproaching God, we may ask him to help us to silence them by sure proofs of his mercy and faithfulness.

For I trust in thy word. His faith was seen by his being trustful while under trial, and he pleads it as a reason why he should be helped to beat back reproaches by a happy experience. Faith is our argument when we seek mercies and salvation; faith in the Lord who has spoken to us in his word. "I trust in thy word" is a declaration more worth the making than any other; for he who can truly make it has received power to become a child of God, and so to be the heir of unnumbered mercies. God hath more respect to a man's trust than to all else that is in him; for the Lord hath chosen faith to be the hand into which he will place his mercies and his salvation. If any reproach us for trusting in God, we reply to them with arguments the most conclusive when we show that God has kept his promises, heard our prayers, and supplied our needs. Even the most sceptical are forced to bow before the logic of facts.

In this second verse of this eight the Psalmist makes a confession of faith, and a declaration of his belief and experience. Note that he does the same in the corresponding verses of the sections which follow. See Psalms 119:50, "Thy word hath quickened me"; Psalms 119:58, "I entreated thy favour"; Psalms 119:66, "I have believed thy commandments"; Psalms 119:74, "I have hoped in thy word." A wise preacher might find in these a series of experimental discourses.