‘Judge me, O God,
And plead my cause against an ungodly nation,
Oh deliver me from the deceitful and unjust man.
For you are the God of my strength,
Why have you cast me off?
Why go I mourning because of the oppression of the enemy?'
He calls on God to judge him, with a view to vindicating him because of his love for Him, and to plead his cause before the godless nation which holds him. He seeks to be delivered from the hand of the deceitful and unjust men who represent that nation. The singular is signifying unjust man in general. They do not walk in God's ways and therefore God must surely finally deliver him (who does walk in God's ways) from their hands. For He is the God of his strength (and thus his rocky fortress).
So he again asks (compare Psalms 42:9) why God has seemingly cast him off, and allowed him to go in mourning because of the oppression of his enemies. He echoes the words of all those who have found themselves in difficult situations which do not seem to tie in with God's love. And it gives assurance that God does know of our situation. The fact that we have this psalm available demonstrates that eventually he was released or ransomed.