The heart of liberation theology beats throughout the psalms. The Lord is mindful of tears, and his anger waxes hot against those who cause them.
We often describe the Kingdom of God as an "upside down" kingdom, where the least, lowest, and weakest are elevated over the great, powerful, and strong. There is strong continuity between the Old and the New Testaments on this point. With Moses before Pharaoh, with the prophets, and with the Psalms, we see God's concern for the oppressed and for the "quartet of the vulnerable"--the widow, the orphan, the poor and the refugee.
As many scholars and historians have noted, the origins of what we call "social justice" can be traced back to the Hebrew Scriptures. And here in Psalm 9, we see why. The Lord does forget the cry of the oppressed.