(41) Let thy mercies come also unto me, O LORD, even thy salvation, according to thy word.
According to God's Word, He has promised salvation. According to God's Word, He has promised mercy. It is not arrogant or prideful to ask the Lord for what He has promised ("Ask, and it shall be given you; seek, and ye shall find; knock, and it shall be opened unto you: For every one that asketh receiveth; and he that seeketh findeth; and to him that knocketh it shall be opened." Matthew 7:7-8 - "If ye abide in me, and my words abide in you, ye shall ask what ye will, and it shall be done unto you." John 15:7). How many of God's blessings are we missing out because we're not asking? because we're not asking for Him to send them to us?
(42) So shall I have wherewith to answer him that reproacheth me: for I trust in thy word.
God's Word gives the answer for those who test us (as Ephesians 6:17 indicates this weapon, "And take the helmet of salvation, and the sword of the Spirit, which is the word of God:") - but it is only useful if we trust in God's Word. If we doubt His Word, then this weapon is then in vain. The question is not "Has God given us adequate answers?" but "Do I trust in the answers God has given?"