Why is it an embarrassment to God when people add human works to what Christ has done?
It is an embarrassment to God because Christ's death completely fulfilled everything a holy God could demand, so when man says he must still do this or that, he contradicts God's own declaration that it is done. Everything needed has already been accomplished in Christ, not by human works.
Les explains that when Christ died, He so completely fulfilled everything a holy God could possibly demand that God says, "It's done." Therefore, as soon as man tries to say, "But I've got to do this, or I've got to do that," it is an embarrassment to God Himself, because everything we need has already been accomplished. The context also says the way into the holiest of holies was opened, not just to the high priest on the Day of Atonement but to everybody, and the veil was rent from top to bottom. This rests on justification by faith: God's judicial act whereby He declares the sinner just as if he had never sinned.