We caution not to get too caught up on the specific legalities of labeling each sin, as both are God-dishonoring and closely related scripturally with idolatry. Generally, fornication is understood to mean sex between people who are not married to each other. Adultery is sex between a married person and someone other than their spouse. While all adultery is fornication, all fornication is not adultery.
Sexuality has a sacred and honorable place is God’s plan. But when taken out of that sacred place, it becomes evil and dark: Hebrews 13:4: (NASB 1995) Marriage is to be held in honor among all, and the marriage bed is to be undefiled; for fornicators and adulterers God will judge.
In the sight of God, the marriage covenant requires wholehearted commitment, exclusivity, and unswerving loyalty to one’s partner. Jesus adds, So that they are no more two, but one flesh. What therefore God hath joined together, let not man put asunder. Matthew 19:6
Self-control, a fruit of the spirit in Galatians 5, is required. Pre-marital and extra-marital sex is a sin, and marriage is to be held in honor.
Since the Christian marriage is also an earthly picture that symbolizes the heavenly union of Christ and his bride, when marital problems arise it is the obligation of both parties to prayerfully ask the Lord for guidance to find a way to set matters right. When a person strays and engages in fornication outside the marriage covenant, he or she is not respecting God’s sacred laws. Anyone who honors God’s laws and is willing to sacrifice his or her own desires to do God’s will, will not engage in fornication, whether married or single.
There are five words that describe immorality/fornication in the New Testament. They are all related and tell a complete story. Each of these five words have the word “porn” in them, related to our English word “pornography.” They all involve walking away from God – being idolatrous.
Such immorality – like all sin – eventually leads to a dead end of godless actions. Fornication is normalized and even encouraged in our world today, but that does not change what it means to God! While we have the free will choice to do and say whatever we want, we are either acting in accordance with God’s sacred standards or we are running far away from them.
No matter where you look in the Bible, fornication is a heinous sin before God. It violates the sacredness and holiness of any Christian’s life. To confess, repent and ask forgiveness is required.
Can we fall from grace after having been called to be sons of God and heirs with Jesus and after having been enlightened by the holy spirit? Yes, it is possible if those who have dedicated their lives to following Christ then renounce the sacrifice of Jesus and go back into the world to sin (Hebrews 6:4-6). The path without repentance and change leads to what the Bible calls “second death” (2 Peter 2:22).
However, our God is merciful and will accept our sincere repentance for these kinds of sins. (1 John 1:9). The Parable of the Lost Sheep (Luke 15:4-7) teaches us that the extent of God’s love and desire to save every lost “sheep.” When one is recovered, not only does God rejoice, but all the angels rejoice with Him.