After Pentecost, when 3,000 people were to be saved and baptized, they continued in fellowship. “Then they that gladly received his word were baptized: and the same day there were added unto them about three thousand souls. And they continued stedfastly in the apostles’ doctrine and fellowship, and in breaking of bread, and in prayers.” (Acts 2:41-42, KJV).
Paul repeatedly instructed that we are to continue in this fellowship. If we are in right fellowship with Jesus, we will be in right fellowship with each other. “God is faithful, by whom ye were called unto the fellowship of his Son Jesus Christ our Lord. Now I beseech you, brethren, by the name of our Lord Jesus Christ, that ye all speak the same thing, and that there be no divisions among you; but that ye be perfectly joined together in the same mind and in the same judgment.” (I Cor 1:9-10).
He was not speaking about being in fellowship with everyone and everything. We are not to try to be in fellowship with those who are working against God on an outwardly basis. We can try to teach them the Gospel, but we are not to participate in their worship of foreign gods and/or idols. “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.” (I Cor 10:19-21).
This is why Paul said that we cannot be part of two religions — one that worships God and one that worships idols or foreign gods. “Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness?” (II Cor 6:14).
We are to speak truthfully and reprove those who work against Christ Jesus. “And have no fellowship with the unfruitful works of darkness, but rather reprove them. For it is a shame even to speak of those things which are done of them in secret.“ (Eph 5:11-12).
John wrote that to have fellowship with God the Father through Jesus Christ His Son was to also have fellowship with each other. “That which we have seen and heard declare we unto you, that ye also may have fellowship with us: and truly our fellowship is with the Father, and with his Son Jesus Christ. And these things write we unto you, that your joy may be full. This then is the message which we have heard of him, and declare unto you, that God is light, and in him is no darkness at all. If we say that we have fellowship with him, and walk in darkness, we lie, and do not the truth: But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin.” (I John 1:3-7).
As John said, we are to declare the truths of Christ always. We are to love those with whom we disagree, but not be silent and allow them to think we are in agreement when their actions are against the very Word of God.