“In common use ‘fellowship’ has become somewhat debased. If you invite a pagan neighbour to your home for a cup of tea, it is friendship; if you invite a Christian neighbour, it is fellowship. If you attend a meeting at church and leave as soon as it is over, you have participated in a service; if you stay for tea and crumpets afterward, you have enjoyed some fellowship. In modern use, then, fellowship has come to mean something like warm friendship with believers.
In the first century, however, the word commonly had commercial overtones. If John and Harry buy a boat and start a fishing business, they have entered into a fellowship, a partnership. Intriguingly, even in the New Testament the word is often tied to fiscal matters. Thus when the Macedonian Christians send money to help the poor Christians in Jerusalem they are entering into a fellowship with them.
The heart of true fellowship is self-sacrificing conformity to a shared vision. Both John and Harry put their savings into the fishing boat. Now they share the vision that will put the fledgling company on its feet. Christian fellowship, then, is self-sacrificing conformity to the gospel. There may be overtones of warmth and intimacy, but the heart of the matter is this shared vision of what is of transcendental importance, a vision that calls forth our commitment. So when Paul gives thanks, with joy, because of the Philippians’ ‘partnership in the gospel’ or ‘fellowship in the gospel’, he is thanking God for the fact that these brothers and sisters in Christ, from the moment of their conversion (‘from the first day until now’, Paul writes), rolled up their sleeves and got involved in the advance of the gospel. They continued their witness in Philippi, they persevered in their prayers for Paul, they sent money to support him in his ministry – all testifying to their shared vision of the importance and priority of the gospel.
D.A. Carson, Basics for Believers. (Leicester, IVP, 2004) p.13-14