This passage is one of my favorite, and also one of the most terrifying, passages in all of Isaiah. It is a favorite of mine because it calls us back to the simple good works that ought to be the natural overflow of every Christian's life: "Remove your evil deeds from my sight. Stop doing evil. Learn to do what is good. Pursue justice. Correct the oppressor. Defend the rights of the fatherless. Plead the widow’s cause" (Isaiah 1.16-17 CSB). These same concerns are repeated throughout the Prophets, and James echoes them in the New Testament. As St. John Chrysostom points out, "Do you see the great importance God places on mercy and of standing up for those who have been treated unjustly? We should pursue these good works, and by the grace of God will we receive the blessings to come." The actions of our faith are anything but just going through the motions of our liturgy (vv. 11-14). Instead, they are a faith lived out in good works, righteous living, and caring for the vulnerable ones among us.
This passage is terrifying because of God's response to the loveless faith of Israel is to ignore their prayer, refuse their sacrifices, and leave them to themselves until they come to repentance. I imagine Israel worshiping earnestly (so they think) and according to the revelation of the Old Covenant, aware of their cold hearts but comparing themselves to the nations and faiths around them, aware of their own sins but justifying or assuaging themselves because they aren't "that bad" compared to those around them...all the while precariously unaware that God has turned aside from them because of their hypocrisy, pride, intentional sins, and unwillingness to repent. I am no prophet, but I see echoes of Israel's condition in the American church (both among evangelical and mainlines for their own reasons), where we too are confident in our 'status' as Christians apart from a characterization and demonstration of hearts transformed by the grace of God. Kyrie eleison!