What is the fate of a child born from an adulterous relationship with a Christian girl who refuses marriage, and is the father accountable for the sin of raising him outside of Islam, and how can he repent and be saved from this sin?

Adultery is one of the most heinous sins and greatest major transgressions, but Allah accepts repentance and rejoices in it, transforming evil deeds into good ones. Repentance involves abandoning the sin, regretting it, and resolving not to return to it, while concealing one's fault and not publicizing it. The child born from adultery is not attributed to you but to its mother. You must strive to perform good deeds, hasten to marry, and frequently engage in the remembrance of Allah and supplication to Him.