The English word communion means fellowship, or participation in something belonging to all, a "common union." It's used in the Holy Bible (see 1 Year Holy Bible Reading Plan) to translate the Greek word (pronounced) coy-nohn-ee-ah which means to associate, or to participate in a partnership, as in parts that make up a whole. The original Greek word is translated (very accurately and appropriately) much more often as fellowship.
Fellowship of Believers
The apostle Paul (see Paul's Ministry) used "communion," that is, the Greek word coy-nohn-ee-ah, during the once per year observance of the Passover (see Unleavened Bread), in describing how individual Believers compose the "body of Christ." In contrast, Paul also described the pagan "communion" that is observed on altars of devilish idolatry:
"The cup of blessing which we bless, is it not the communion of the blood of Christ? The bread which we break, is it not the communion of the body of Christ?" For we being many are one bread, and one body: for we are all partakers of that one bread."
"Behold Israel after the flesh: are not they which eat of the sacrifices partakers of the altar? What say I then? that the idol is any thing, or that which is offered in sacrifice to idols is any thing? But I say, that the things which the Gentiles sacrifice, they sacrifice to devils, and not to God: and I would not that ye should have fellowship with devils. Ye cannot drink the cup of the Lord, and the cup of devils: ye cannot be partakers of the Lord's table, and of the table of devils." (1 Corinthians 10:16-21 KJV)
Paul again uses "communion" to describe the fellowship of believers:
"Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? And what communion hath Light with darkness? And what concord hath Christ with Belial? or what part hath he that believeth with an infidel?"
"And what agreement hath the Temple of God with idols? For ye are the Temple of the living God [see Physical and Spiritual Temples, also Physical and Spiritual Israel and Daughter of Zion]; as God hath said, I will dwell in them, and walk in them; and I will be their God, and they shall be My people." (2 Corinthians 6:14-16 KJV)
In other verses, the very same original Greek word translated as "communion" is also rendered, more plainly, as "fellowship":
"And they continued stedfastly in the apostles' doctrine [see also Doctors Of The Law] and fellowship [i.e. "communion"]" (Acts 2:42 KJV)
"God is faithful, by whom ye were called unto the fellowship [i.e. "communion"] of his Son Jesus Christ our Lord." (1 Corinthians 1:9 KJV)
"And when James, Cephas, and John, who seemed to be pillars, perceived the grace that was given unto me, they gave to me and Barnabas the right hands of fellowship [i.e. "communion"]" (Galatians 2:9 KJV)
"And to make all men see what is the fellowship [i.e. "communion"] of the Mystery, which from the beginning of the world hath been hid in God, who created all things by Jesus Christ" (Ephesians 3:9 KJV)
"That which we have seen and heard declare we unto you, that ye also may have fellowship [i.e. "communion"] with us: and truly our fellowship is with the Father, and with his Son Jesus Christ." (1 John 1:3 KJV)
"But if we walk in the Light, as He is in the Light, we have fellowship [i.e. "communion"] one with another, and the blood of Jesus Christ His Son cleanseth us from all sin [see Why Blood?]" (1 John 1:7 KJV)
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