The original word for fellowship is "joint participation." Another word that describes fellowship is "partners" (cf. Luke 5:7, 10). These disciples were partners in the occupation of fishing, but we are not talking about fishing. We are talking about what the word fellowship means and how it is used in the Scriptures. The church in Jerusalem "...continued stedfastly in the apostles' doctrine and fellowship, and in breaking of bread, and in prayers" (Acts 2:42).
If we can ascertain what the apostles' doctrine was/is we will know the answer the question posed above. Jesus said, "For whosoever shall be ashamed of me and of my words, of him shall the Son of man be ashamed, when he shall come in his own glory, and in his Father's, and of the holy angels" (Luke9:26). Jesus said, “My words.” Were the words He spoke His own? No they were not! Jesus plainly said, "For I have given unto them the words which thou gavest me; and they have received them, and have known surely that I came out from thee, and they have believed that thou didst send me" (John 17:8). Now, please notice with me the following. The apostles' words or teaching and the words or teaching of Christ was received by Christ from God the Father. They were not His words in that they originated with Him, but the very Words of God – the words God gave Him. Therefore, the apostles' doctrine was the doctrine of Christ which was the doctrine that God gave Him and He to the apostles. Anyone who can see through a ladder can see this.
Let us back up to Acts 2:42 and ask, was one not following the apostles' doctrine in fellowship with them? A resounding "No!" Okay, was one not following the apostles' doctrine in fellowship with Christ? Again the answer is "No" We ask again, was one who was not following the apostles' doctrine (teaching) and the doctrine (teaching) of Christ, in fellowship with God? Again, the answer is absolutely "No" Since the saved were added to the church (cf. Acts 2:47) those who did not obey the apostles doctrine would not be in the church would they? "No!" There were no saved ones outside the body of Christ. One may ask, what if one had obeyed the Gospel and was added to the church but followed not in the doctrine of the apostles, Christ and God, are they in fellowship with the faithful in the Lord's church? "No!"
Did not Paul and other inspired writers warn the church about fellowship practices? Of course they did! Consider Paul's letter to the Ephesians when he commanded, "And have no fellowship with the unfruitful works of darkness, but rather reprove them" (Ephesians 5:11). We wish to ask, was this Paul's doctrine alone? No, it was the doctrine of Christ. Was it the doctrine of Christ alone? No, it was the doctrine of God. What is/are "works ofdarkness?" This is any work or practice that is not authorized in God's eternal Word. How, then, can we fellowship those who do not follow the teachings of Christ?
What about those who bid God speed to one who does not abide in the teachings of Christ (cf. II John 9-11)? The Scriptures say that such a one does not have either the Father or the Son? Someone might say, "Brother so and so does not believe the error nor does he teach the error of the false teacher. Why he only speaks on the same programs with the false teacher." God says in essence that such a one has left God behind.