This was his response when asked whether believers should attend the wedding of a couple who cohabit before marriage.

According to Piper, who is a pastor, author, founder of desiringGod.org and chancellor of Bethlehem College & Seminary in Minnesota, Christians should stop supporting couples who believe it is okay to fornicate before marriage even though the Bible says fornication is a sin.

In his words, "It is clear from Scripture that not only is adultery - sexual unfaithfulness in marriage - sin, but sexual relations before marriage is sin as well. That's clear. The very term sexual immorality as it's used in these verses makes it clear: Matthew 15:19; Hebrews 13:4; Ephesians 5:5; 1 Corinthians 5:11; 1 Corinthians 6:9; 7:1–2. All refer to fornication or sexual relations before marriage."

"Many in our day, tragically, are deluded about this because of how superficially they submit to Scripture. They don't submit themselves to God's authority in Scripture. They just do what they feel like doing, and assume God is okay with it - like sleeping together before they're married because they think they're committed to each other."

He added, "If they are professing Christians getting married, that makes the relationship all the more difficult and complicated since the Bible says we are to disassociate from brothers, professing brothers, who live in this kind of sin (1 Corinthians 5:11)."

ALSO READ: Is the church the best place for Christians to discuss sex?

Why  Christians who have pre-marital sex need help

Piper goes on to explain why this is so important adding that behavior like fornication "is like murder - murder in our heart."

He explained, "'Those who do such things will not enter the kingdom of heaven,' Paul says (1 Corinthians 6:9). He's talking about fornication, not just homosexuality. What complicates the question being asked, then, is that if the couple approves of fornication - they don't think it's wrong for an engaged couple to have sex - and they are professing Christians, then the Bible says, 'Now I am writing to you not to associate with anyone who bears the name of brother if he is guilty of sexual immorality . . . not even to eat with such a one' (1 Corinthians 5:11)."

This is why Piper says Bible-believing Christian couples who think that premarital sex is permissible need to be "disciplined."

"If the couple that we're talking about here, whose wedding you're going to attend, has only stopped doing the act of fornication, but has not stopped believing that fornication is right, then they probably (if they belong to a Bible-believing church) are in a position where they should be disciplined - because we don't just discipline people for unrepentant actions of sinning, but also for unrepentant belief that sin is right or permissible," he noted.

On the other hand, Piper says he is not stopping people from attending weddings of Christian couples who have truly repented before marriage.

"If they are moving away from fornication because they are now persuaded it is sin, and they are marrying as a declaration of repentance and faith in Christ and a commitment to righteousness, then they are right with God. We should join them in the penitent and happy celebration," Piper said.