Psalm 143 is the fourth in the sequence of five psalms of David (within the collection of eight) wherein David prays for deliverance from enemies. It is classified as one of the seven penitential psalms (Psalms 6; 32; 38; 51; 102; 130; 143). These psalms have in common an acknowledgment of sin (Psalms 32:5 Psalms 32:5I acknowledge my sin to you, and my iniquity have I not hid. I said, I will confess my transgressions to the LORD; and you forgave the iniquity of my sin. Selah.
American King James Version×; Psalms 8:18 Psalms 8:18
American King James Version×; Psalms 51:2-4 Psalms 51:2-4  Wash me thoroughly from my iniquity, and cleanse me from my sin.  For I acknowledge my transgressions: and my sin is ever before me.  Against you, you only, have I sinned, and done this evil in your sight: that you might be justified when you speak, and be clear when you judge.
American King James Version×; Psalms 130:3 Psalms 130:3If you, LORD, should mark iniquities, O Lord, who shall stand?
American King James Version×) and/or a reference to deserved punishment (Psalms 6:1 Psalms 6:1O LORD, rebuke me not in your anger, neither chasten me in your hot displeasure.
American King James Version×; Psalms 38:1 Psalms 38:1O lord, rebuke me not in your wrath: neither chasten me in your hot displeasure.
American King James Version×; Psalms 102:10 Psalms 102:10Because of your indignation and your wrath: for you have lifted me up, and cast me down.
American King James Version×; Psalms 143:2 Psalms 143:2And enter not into judgment with your servant: for in your sight shall no man living be justified.
American King James Version×).
In this psalm, David doesn't acknowledge specific sins but is clearly aware of his own failings, asking to be passed over in judgment. He knows that such judgment would find him guilty, as he, like everyone, has sinned (compare Psalms 143:2 Psalms 143:2And enter not into judgment with your servant: for in your sight shall no man living be justified.
American King James Version×; Romans 3:10 Romans 3:10As it is written, There is none righteous, no, not one:
American King James Version×, Romans 3:23 Romans 3:23For all have sinned, and come short of the glory of God;
American King James Version×). So he pleads for mercy, basing his appeal on God's faithfulness and righteousness (Psalms 143:1 Psalms 143:1Hear my prayer, O LORD, give ear to my supplications: in your faithfulness answer me, and in your righteousness.
American King James Version×) in dealing with one who is His servant (verse 2; compare verses 11-12).
David further implies that he is unable to withstand judgment given his already-overwhelmed state, crushed to the depths of despair by enemy persecution that brought him seemingly near death (verses 3-4, 7). Although David is probably referring to a human enemy (verse 3) and enemies (verse 12) who have persistently hounded him, he may have in mind as well the spiritual Adversary, Satan the devil, and his demons, who are also associated with darkness and the pit (see Ephesians 6:12 Ephesians 6:12For we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places.
American King James Version×; Revelation 20:1-2 Revelation 20:1-2  And I saw an angel come down from heaven, having the key of the bottomless pit and a great chain in his hand.  And he laid hold on the dragon, that old serpent, which is the Devil, and Satan, and bound him a thousand years,
American King James Version×).
David likens his yearning for God to dry ground that needs rain (verse 6). Interestingly, rain in other passages symbolizes God's Word and teachings (Isaiah 55:6-13 Isaiah 55:6-13  Seek you the LORD while he may be found, call you on him while he is near:
 Let the wicked forsake his way, and the unrighteous man his thoughts: and let him return to the LORD, and he will have mercy on him; and to our God, for he will abundantly pardon.
 For my thoughts are not your thoughts, neither are your ways my ways, said the LORD.
 For as the heavens are higher than the earth, so are my ways higher than your ways, and my thoughts than your thoughts.
 For as the rain comes down, and the snow from heaven, and returns not thither, but waters the earth, and makes it bring forth and bud, that it may give seed to the sower, and bread to the eater:
 So shall my word be that goes forth out of my mouth: it shall not return to me void, but it shall accomplish that which I please, and it shall prosper in the thing whereto I sent it.
 For you shall go out with joy, and be led forth with peace: the mountains and the hills shall break forth before you into singing, and all the trees of the field shall clap their hands.
 Instead of the thorn shall come up the fir tree, and instead of the brier shall come up the myrtle tree: and it shall be to the LORD for a name, for an everlasting sign that shall not be cut off.
American King James Version×; Deuteronomy 32:1-3 Deuteronomy 32:1-3  Give ear, O you heavens, and I will speak; and hear, O earth, the words of my mouth.  My doctrine shall drop as the rain, my speech shall distil as the dew, as the small rain on the tender herb, and as the showers on the grass:  Because I will publish the name of the LORD: ascribe you greatness to our God.
American King James Version×), the Holy Spirit (Isaiah 44:3-4 Isaiah 44:3-4  For I will pour water on him that is thirsty, and floods on the dry ground: I will pour my spirit on your seed, and my blessing on your offspring:  And they shall spring up as among the grass, as willows by the water courses.
American King James Version×), righteousness (Hosea 10:12 Hosea 10:12Sow to yourselves in righteousness, reap in mercy; break up your fallow ground: for it is time to seek the LORD, till he come and rain righteousness on you.
American King James Version×) and the coming of God (Hosea 6:1 Hosea 6:1Come, and let us return to the LORD: for he has torn, and he will heal us; he has smitten, and he will bind us up.
American King James Version×, Hosea 6:3 Hosea 6:3Then shall we know, if we follow on to know the LORD: his going forth is prepared as the morning; and he shall come to us as the rain, as the latter and former rain to the earth.
American King James Version×). David needs all of this. In desperation he cries out for God's immediate intervention. He cannot rely on his own overwhelmed and failing spirit (Psalms 143:4 Psalms 143:4Therefore is my spirit overwhelmed within me; my heart within me is desolate.
American King James Version×, Psalms 143:7 Psalms 143:7Hear me speedily, O LORD: my spirit fails: hide not your face from me, lest I be like to them that go down into the pit.
American King James Version×), referring to his weakened strength of mind. He requires the help of God's good Spirit (see verse 10). He needs an understanding of how to go forward (verse 8), rescue from his enemies (verse 9), instruction in righteousness (verse 10), and relief and empowerment (verse 11).
As in the opening of the psalm (verses 1-2), David again bases his plea for help (including justice on enemies) on the fact that he is God's servant (verses 11-12)—stressing here God's hesed, rendered "mercy" (verse 12, NKJV) but also translatable as "loyal love" or "devotion." The point is that God has made promises of steadfast love and help to those who are His servants—even, as verse 11 implies, staking His name, His reputation, on this.