Claim. The reason God cannot forgive Satan — though scripture says God can forgive even murder — is that Satan is the adulterer who defiled the lineage of humankind, an offense that cannot be remitted by forgiveness alone but only by re-rooting.
Elaboration. SMM frames the unforgivability of Satan as a lineage problem: “The reason God cannot forgive Satan is because he defiled the blood line of humankind. … From God’s point of view, Satan is the adulterer, the adulterer of love” (1.3. Reasons for receiving the Blessing, 156-226, 1966.5.25). The rhetorical pivot is repeated throughout the section: “Why do people do penance by inflicting pain on their bodies? This is because they have inherited the blood of Satan, the blood of the enemy of God. Who is this enemy of God? He is the adulterer” (ibid, 172-277, 1988.1.24). Satan, originally “a servant of God,” violated “the daughter of his master” (ibid, 85-308, 1976.3.4). The implication is that forgiveness-language alone cannot address the fall — only a change-of-lineage through True Parents can.
See also. fall, original-sin