In Psalm 19, we read of two great witnesses to the glory of God: the heavens and His Holy Law. Both are sources through which we can come to know God. The first is a universal witness which brings mankind to believe in the existence of God. The second leads one to a deeper and more personal revelation of God.
All over the earth, God's beautiful creation bears witness to His glory. "Day unto day" and "night unto night" reveals knowledge of Himself (19:2). The course of nature continually attests to God's existence and majesty. The bright shining sun God created is considered a great agent in revealing His glory; its intense light and energy is but a token of God's power.
After contemplating God's glory in the heavens, the psalmist turns to consider the glory of God as revealed in His law, testimony, statutes, commandments, and judgments, which are altogether righteous, pure, and enduring forever (19:9). This is not only referring to the Law of Moses (Torah) but the totality of God's revelation through the Holy Spirit-inspired scriptures. The tremendous effects of God's Word upon mankind include: restoring the soul, making wise the simple, rejoicing the heart, and enlightening the understanding (19:7-8). The excellence of God's Word surpasses even the most excellent and desirable things on the face of the earth, such as the purest gold or sweetest honey. The Scripture is the most beneficial thing for mankind. Consider how important it is to read, learn, and practice it! God's Word warns us so we might not see destruction and eternal death, and by faithfully following it, we will be rewarded with God's pleasure upon us.
Knowing the Bible, however, makes mankind aware of his depravity, sinfulness, and guilt, especially when compared to God's righteousness and holiness. David's prayer (19:12-14) is exemplary, and it is echoed in the prayer that Jesus taught us to pray: "And do not lead us into temptation, but deliver us from the evil one" (Matthew 6:13). Mankind needs to continually pray for God's strength and sanctification, for although he can never be sinless, he can strive to be blameless by not allowing sin to have dominion over his life. When one has been redeemed by the Lord and is living victoriously over sin, he can be sure that his prayers will be acceptable in God's sight (19:14).
Psalm 20 is addressed to the king before going into battle and is also a prayer for divine intervention on behalf of the king. It would have been sung as a responsive hymn with the participation of the Levitical choir (20:1-5), the king or high priest (20:6-8), and the congregation (20:9). The petition was for the Lord to go before them into battle and grant them victory. Their battles were fought in the Name of the Lord God of Jacob (20:1, 5), and so when they went out to war, God's name and reputation as a deliverer was at stake. The intent of their prayer, then, was also to invoke God to prove Himself once again to be their Deliverer.
It was their practice to make sacrifical offerings to God before going to war (20:3), and if their offerings were acceptable unto God, then they could be confident that God would be with them to give them victory. Of course, this could not be done as a means to manipulate God, for that is impossible; the outcome was totally dependent upon the will of God (20:4).
There appears to be an interval of time between verses five and six; this was possibly the time for the offering of the sacrifices. Once they were completed, the king or high priest spoke in a prophetic manner to acknowledge that God had accepted the sacrifice and that He would be their saving strength. The speaker knew beyond any doubt that "the Lord saves His anointed" (20:6). Their enemy would put their trust in the most modern of war machines (chariots and horses), but that was in vain, for "they have... fallen". Notice that he declares their defeat as a fact of the past, even though the battle had not yet begun. The Lord's people can put their trust solely in God and be assured of the victory (20:8). Before marching to war, the whole congregation, already confident of the answer, prayed, "Save the king! Answer us when we call!" (N.I.V., 20:9).