Could you please explain 1 Corinthians 6:18Flee fornication. Every sin that a man doeth is without the body; but he that committeth fornication sinneth against his own body.
See All... - "Flee fornication. Every sin that a man doeth is without the body; but he that committeth fornication sinneth against his own body."

Initially, we need to define fornication. It has both a narrow and a broad meaning and both are used in the Bible. In its narrow meaning, it refers specifically to physical relations between two unmarried people. However, in its broader meaning (its most common usage in scripture), it refers to any unlawful sexual union. As such, it includes the narrow definition of fornication, as well as adultery, sodomy, and other perverse unions. It is almost certain that the broader meaning of the word is in context here.

1 Corinthians 6:18Flee fornication. Every sin that a man doeth is without the body; but he that committeth fornication sinneth against his own body.
See All... tells us that fornication is different from all other sins in that the one who commits this sin "sinneth against his own body." This is a difficult passage for numerous reasons. Some have said that this refers to the fact that this sin directly harms the body whereas other sins do so only indirectly. That may be true of stealing or lying, but it is not true of drunkenness or drug abuse. Surely, they directly harm the body as well. Perhaps the ultimate harming of the body comes from suicide. Yet, this verse tells us that fornication sins against the body in a way that suicide does not. Others had taught that since marriage joins two into one body, the man who commits adultery is sinning against his wife. But marriage does not seem to be the emphasis here and fornication can be committed by those who are not married. Let us consider a solution to the problem. But first, this verse cannot be understood apart from the passage in which it is found.

1 Corinthians 6:15Know ye not that your bodies are the members of Christ? shall I then take the members of Christ, and make them the members of an harlot? God forbid.
See All... Know ye not that your bodies are the members of Christ? shall I then take the members of Christ, and make them the members of an harlot? God forbid. 16 What? know ye not that he which is joined to an harlot is one body? for two, saith he, shall be one flesh. 17 But he that is joined unto the Lord is one spirit. 18 Flee fornication. Every sin that a man doeth is without the body; but he that committeth fornication sinneth against his own body. 19 What? know ye not that your body is the temple of the Holy Ghost which is in you, which ye have of God, and ye are not your own? 20 For ye are bought with a price: therefore glorify God in your body, and in your spirit, which are God's.

1. Our bodies are "the members of Christ" (1 Corinthians 6:15Know ye not that your bodies are the members of Christ? shall I then take the members of Christ, and make them the members of an harlot? God forbid.
   See All...). Ephesians 5:30For we are members of his body, of his flesh, and of his bones.
   See All... states, "For we are members of his body, of his flesh, and of his bones." 1 Corinthians 12:27Now ye are the body of Christ, and members in particular.
   See All... declares, "Now ye are the body of Christ, and members in particular." We who are saved have a unique and integral connection with Jesus Christ. Our own bodies are members of Him.
2. When we are "joined to the Lord" in salvation, we are "one spirit" with Him (1 Corinthians 6:17But he that is joined unto the Lord is one spirit.
   See All...). Although this deals with the spiritual aspect of our salvation, it is connected to the physical nature of the believer. Our physical body takes on some level of spiritual significance because it houses the spirit that is joined with the Lord.
3. Our body becomes "the temple of the Holy Ghost" (1 Corinthians 6:19What? know ye not that your body is the temple of the Holy Ghost which is in you, which ye have of God, and ye are not your own?
   See All...) by His indwelling. Whatever building the Spirit of the Lord inhabits; that is the temple of the Lord.
4. Our body, as well as our spirit, has been bought with a price (Christ on the cross) and truly belongs to the Lord.

Bringing an idol into God's holy temple and worshipping it there or sacrificing a pig on the altar of the Lord (as done by Antiochus Epiphanes about 200 years before the crucifixion) is about the most defiling attack that can be made on the temple. These acts would be seen as direct attacks on God's temple, joining God's holy dwelling place with false gods. In like manner, our body is the temple of the Lord and joining it with an harlot is a direct attack on that holy temple. It is a spiritual attack directly against the dwelling place of the Holy Ghost.

I think the significance is seen in that no other sin attacks the very concept of our body being the temple of the Lord as this sin. They are outside the body. Some sins, such as drunkenness, may harm the body, but they are still not a direct spiritual attack on the sanctity of the body as fornication is. Fornication defiles the Christian's own holy temple. He attacks himself in a direct and terrible way when he commits this sin.

This is indeed a difficult verse. However, I hope that this helps you see some of its significance. We take our bodies lightly and count them as unimportant. Evidently, God does not.