Unsearchable Riches

Recently in reading Ephesians I got stuck on this verse, Eph 3:8 Unto me, who am less than the least of all saints, is this grace given, that I should preach among the Gentiles the unsearchable riches of Christ; wondering how rich is God's mercy. We tend to measure riches by some standard - how does God measure it. He says "unsearchable".
Many a man has set himself on the mission to determine, Where is the end of God's mercy?
A) Some would say murder, but Moses the murderer would say Psa 103:7-8 He made known his ways unto Moses, his acts unto the children of Israel. The LORD is merciful and gracious, slow to anger, and plenteous in mercy.
B) Some would say adultery, but all David the adulterer had to say was:
"Have mercy upon me, O God, according to thy loving kindness: according unto the multitude of thy tender mercies blot out my transgressions" Psalm 51:5
"For great is thy mercy toward me: And thou hast delivered my soul from the lowest hell." Psa 86:13
C) Some would say, when you begin to backslide, but Psa 94:18 says: "When I said, my foot slipeth: thy mercy, O Lord, held me up."
D) Some would say denial, but Peter said: I Pet 1:3-5.
E) Some would say failure to repent: I Corinthians 3:5
F) But still a voice of a protester suggest that there must be somewhere an eternally vile sinner whose wretchedness cannot be touched by God's mercy.
Consider John Newton, Slave trader:
"I am not what I ought to be! How imperfect and deficient I am! I am not what I wish to be, although I abhor that which is evil and would cleave to what is good! I am not what I hope to be, but soon I shall be out of mortality, and with it all sin and imperfection. Though I am not what I ought to be, nor what I wish to be, nor yet what I hope to be, I can truly say, I am not what I once was: A slave to sin and Satan. I can heartily join with the apostle and acknowledge that by the grace of God I am what I am!"