Jonathan Edwards wanted to call his parishioners back to the orthodoxy of the faith, but the whole of his message was to present God as holy yet merciful. What metaphor does he use to show that man deserves to die, but that God's mercy keeps him from falling?

There are so many metaphors that can be used if he is looking at the concept of mercy from the Bible. In the book of Roman 3 vs 23. "for all have sinned and fall short of the glory of God (New King James Version (NKJV))." This shows that every human being is not perfect in one way or the other. If God was to consider the sins of human to punish them, no human will be alive today. This is because the same bible describe the righteousness of human as a filthy rag before God. However, in the next verse of the same Romans 3 vs 24, it reads:

" and are being justified [declared free of the guilt of sin, made acceptable to God, and granted eternal life] as a gift by His [precious, undeserved] [a]grace, through the redemption [the payment for our sin] which is [provided] in Christ Jesus (Amplified Bible (AMP))." From this verse, it therefore signify the amount of mercy humans have received from God. This verse made it clear that it is undeserved (That's why they are bolded for emphasis). So man deserved to die due to sin, however, the death of the son of God (Jesus Christ) on the cross has paved way for redemption and mercy.