These are good questions, but first it is important that we realize that the “Church” and the “Kingdom of Heaven” are not the same. The “church” the Bible speaks of “is made up of all those who believe in Christ” as Saviour, because we are told that “The Lord added to the church daily those who were being saved” (Acts 2:47, NKJV). But “the Kingdom of Heaven” is composed of everyone today (when the King, Jesus, is in heaven) who makes a profession by baptism, church attendance, etc. of believing in Christ whether they are truly saved or not. The Lord shows this in the Parable of the Sower in Matthew 13 where the Lord says the kingdom of heaven is like four types of ground that represent those who hear the good seed of the Word of God, but only one of the four types is a “good ground” hearer of the Word and bears fruit. The other three types of ground bear no fruit for God and therefore represent those who do not receive the Word that is presented to them, and therefore are not saved, but they are also part of the kingdom of heaven.
Notice the ten different things the Lord says the Kingdom of heaven is like:
1. “A man which sowed good seed in his field” (Matthew 13:24).
2. “A grain of mustard seed, which a man took, and sowed in his field” (Matthew 13:31).
3. “Unto leaven, which a woman took, and hid in three measures of meal, till the whole was leavened” (Matthew 13:33).
4. “A merchant man, seeking goodly pearls” (Matthew 13:45).
5. “A net, that was cast into the sea, and gathered of every kind” (Matthew 13:47).
6. “A man that is an householder, which bringeth forth out of his treasure things new and old” (Matthew 13:52).
7. “A certain king, which would take account of his servants” (Matthew 18:23).
8. “A man that is an householder, which went out early in the morning to hire labourers into his vineyard” (Matthew 20:1).
9. “A certain king, which made a marriage for his son” (Matthew 22:2).
10. “A man traveling into a far country, who called his own servants, and delivered unto them his goods” (Matthew 25:14).
Understanding the difference between “the church” and “the kingdom of heaven” will help us know the answer to the questions: “How come we only break bread with so few people each week? How come we don’t participate in activities with other denominations?”
The Lord gave the local assembly or church in Matthew 18:17-20 the responsibility to judge who should be part of the “two or three” (or two or three hundred as the case may be) who represent the church. When someone desires to be part of the local church and express his membership by partaking of the Lord’s Supper, those already in the local church are to make sure that the person shows he is saved in that he lives a Godly life and does not hold teaching that is contrary to the Bible. The words, “A little leaven [yeast] leaveneth the whole lump” are used to show the defiling effect on the whole assembly through one person living immorally (1 Corinthians 5:6) and teaching false doctrine (Galatians 5:9). This person is also not to be associated with anything wrong (1 Corinthians 15:33 and 2 Corinthians 6:14) and at the same time express he is saved and part of the church by partaking of the Lord’s Supper.
Therefore only those who are saved are to be given the privilege of partaking of the bread and wine at the Lord’s Supper. When a person partakes of the cup he is expressing that he has “communion” or fellowship with the blood of Christ (1 Corinthians 10:16). Only those who are saved have fellowship with “the blood of Jesus Christ God’s Son,” which has cleansed “from all sin” those who trust Christ (1 John 1:7). And only those who are saved have communion with the bread which speaks of “the body of Christ” (the church), which is composed of every true believer on Christ: “For we being many are one bread, and one body: for we are all partakers of that one bread” (1 Corinthians 10:17). So if a person is not saved and would partake of the emblems at the Lord’s Supper, he would be expressing a lie.
Regarding the second question: “How come we don’t participate in activities with other denominations?” let us not forget that the Lord wants us to show His love by going “into the all the world and preach the gospel” (Mark 16:15), but He also desires that we find our fellowship by gathering around Himself with other believers who are walking Godly lives: “Where two or three are gathered unto My name there am I in the midst” (Matthew 18:20).
There are many true believers, who are walking Godly lives, who are in the religious denominations. But most denominations allow unsaved people to “take communion.” And the Lord does not want us to be yoked in a common purpose with unbelievers because we really have nothing in common with unbelievers in a religious sense and we also will be defiled if we are yoked with them: “Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? … Wherefore come out from among them, and be ye separate, saith the Lord, and touch not the unclean thing; and I will receive you, And will be a Father unto you, and ye shall be my sons and daughters, saith the Lord Almighty” (2 Corinthians 6:14-18).
This is a lengthy answer but I am praying that the Lord will be pleased to use it to show at least one of His dear people the path God has pointed out in His Word, the Bible, for us to walk in today so we can be part of a testimony representing Christ as Head of His body the church. What a privilege is ours not merely to be separate from the world, but to be separate to Christ. The Lord prayed that this would be true: “Sanctify them through Thy truth: Thy Word is truth” (John 17:17).