Even though the term for fellowship existed before the start of Christianity, the Christian idea of fellowship involves a vastly different motive and goal. Instead of being forced or desiring some immediate earthly gain, the motive of this fellowship is the love of Christ, in that Christians are to love others in the same way that they have been loved by their Savior. Then, instead of wanting to exalt a philosophical system or secular policy, the goal of this fellowship is to highlight the greatness of Jesus Christ and his grace poured out.
This is why the object of fellowship is so important; what do Christians fellowship in? You can find references in the NT which speak of fellowship in the Lord, fellowship in the gospel, and fellowship in the faith. In all of those instances, a mutual sharing with others is inferred. As we have seen on multiple occasions in the book of Acts, the Spirit’s work is a package deal, in that when He brings someone into union with Christ, He baptizes that same person into union with Christ’s body.
In spite of this, there are so many who claim to have fellowship with Jesus Christ and yet have very little fellowship with the body. Other than sharing some time together on Sunday mornings, there is little to no sharing of life. But what the book of Acts supplies is not only the doctrinal convictions of the early church regarding salvation, but also their convictions about the church. These folks had to earn a living through employment, they had children and other family responsibilities, they had to go to the market to purchase food and fix things at their homes, but they also had a demonstrable commitment to sharing life with one another. They shared their time with one another, they shared their resources with one another, they shared their joy with one another and their sorrows. It was stunning unity and God got the glory for what was happening because even the world knew that something other-worldly was taking place.