Psalm 43 contains many of the pleas found in other psalms: the cry for deliverance, the complaint against deceitful, unholy people. What is different, howevr, is verse three: the plea to God to send out His light and His truth.
Unlike other psalms where the oppression of the enemy leads David to pray for God's help in earthly matters, in Psalm 43 David wants only to meet God and to fulfill God's standards. It is as if he has given up on worrying about his enemies. No longer does he care about their triumphing over him. Neither does he want to triumph over them. The only thing he seeks is God and God can be found by God sending the believer spiritual revelation (light) and truth.
If one understands matters spiritually, if one has light and truth that enables God to be seen, one can stand before God's holy altar. With such a vision of God, why should the soul be cast down?
1Judge me, O God, and plead my cause against an ungodly nation: O deliver me from the deceitful and unjust man.
2For thou art the God of my strength: why dost thou cast me off? why go I mourning because of the oppression of the enemy?
3O send out thy light and thy truth: let them lead me; let them bring me unto thy holy hill, and to thy tabernacles.
4Then will I go unto the altar of God, unto God my exceeding joy: yea, upon the harp will I praise thee, O God my God.
5Why art thou cast down, O my soul? and why art thou disquieted within me? hope in God: for I shall yet praise him, who is the health of my countenance, and my God.