What is repentance and how is it performed when one commits major shirk by engaging in adultery with a Christian?

Allah Almighty commanded His servants to hasten to repentance and described those who do not repent as unjust. His (Allah's) repentance is accepted as long as its conditions are met and its impediments are absent, by which Allah erases sins, even if they were like the foam of the sea. Shirk (polytheism) and zina (unlawful sexual intercourse) are among the greatest sins committed by a servant, but Allah accepts the repentance of whoever repents from them. The conditions for sincere repentance are three: abstaining from the sin, regretting having committed it, and a firm resolve not to return to it.