Fornication is "sexual intercourse between two persons not married to each other" or, more broadly, sex other than with someone you can marry, as with incest or adultery.
The Apostle Paul wrote to the Corinthian church:
|| Or do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived; neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor homosexuals, nor thieves, nor the covetous, nor drunkards, nor revilers, nor swindlers, will inherit the kingdom of God.
Such were some of you; but you were washed, but you were sanctified, but you were justified in the name of the Lord Jesus Christ and in the Spirit of our God.
Martin Luther maintained that it is most shameful for Christians and those who are familiar with the Holy Scriptures to incline to the opinion that simple fornication is not a sin.