In the Holy Spirit, that is, when we live in God, we share the lives of other believers, who can be called saints because they are holy people. The tendency of modern reformed churches to translate the Latin sancti as ‘God’s people’ by-passes any reference to their holiness which is certainly not the intention of St. Paul who invented this term. He believed that people who lived in the spirit would be holy, not perfectly and not all at once, but slowly and imperfectly they would separate themselves from the standards of the world, to live in the character of God, as Jesus did. So I would keep the term ‘holy’.
The Latin word ‘communio’ comes from the Greek koinonia, meaning a shared enterprise or shared life. It’s a business word, not holy at all. Paul and John use it to characterise the life of the first church assemblies. Both teach that the life they share is not just that of other believers but the very life of God. This lies behind Paul’s concept of the body of messiah, in which the differently gifted members contribute their unique lives to the life of the whole.
If believers share each other’s lives in the assembly, they also share the lives of the members of all the assemblies, even although they may never meet in person. Paul, as I have mentioned above, was always keen to nurture this sense of communion.
And then of course, as believers died, there grew also a sense that the shared life continued across that barrier, because both the living and the dead believers shared the one life of God. The New Testament authors are careful about this sharing: there is no claim that we can speak with our own dear dead, or consult them about our worldly dilemmas. But we remain in a shared life, albeit separated, until the arrival of God’s new world, where there shall be no separation. And the assemblies may keep special remembrance of their saints, as part of the regular liturgy of the Eucharist, where we “rejoice in the communion of saints.” There is one table although it may exist in different dimensions.
Neglect of the “shared life” of believers on earth and in heaven is common and regrettable, weakening their worship and mission.