So shall I have wherewith to answer him that reproacheth me - I shall have something by which I may reply to those who calumniate me. So the Saviour replied to the suggestions of the tempter almost wholly by passages of Scripture Matthew 4:4, Matthew 4:7,Matthew 4:10; and so, in many cases, the best answer that can be given to reproaches on the subject of religion will be found in the very words of Scripture. A man of little learning, except that which he has derived from the Bible, may often thus silence the cavils and reproaches of the learned sceptic; a man of simplehearted, pure piety, with no weapon but the word of God, may often thus be better armed than if he had all the arguments of the schools at his command. Compare Ephesians 6:17.
For I trust in thy word - I believe it; I rely on it; I confide in that, as my only comfort and protection.

So shall I have wherewith to answer - Many say, "My hope in thy mercy is vain;" but when thou fulfillest thy promises to me, then shall I answer to the confusion of their infidelity.

So shall I (b) have wherewith to answer him that reproacheth me: for I trust in thy word.
(b) By trusting in God's word he assures himself to be able to confute the slanders of his adversaries.

So shall I have wherewith to answer him that reproacheth me,.... Saying there is no help and salvation for him in God; asking where is his God, in whom he trusted? and where is the promise of salvation, on which he depended? To which an easy and ready answer might be given, when the mercies and salvation of God came unto him, and he clearly appeared to be interested in them; see Psalm 3:2;
for I trust in thy word: in Christ the essential Word, the object of trust and confidence; or in the written word, it being divinely inspired and dictated by the Spirit of God, and so to be depended on as true and faithful; or rather God's word of promise concerning mercy, grace, and salvation, which God that has made is faithful and able to perform, as may be believed.

The possession of God's gift of "salvation" (Psalm 119:41) will be the Psalmist's answer to the foe's "reproach," that his hope was a fallacious one.