William Gurnall (d. 1679) has a great paragraph on the fellowship of the saints in his book, The Christian in Complete Armor. In one section, he discusses what he calls “recovery of declining grace.” In other words, if a Christian is not wielding his armor as he should, or if it is getting rusty, what is he to do? How can he grow in grace after languishing? He mentions repentance, faith, and meditation upon Scripture. He also mentions Christian fellowship, the “communion of the saints” in the language of the Creed. I’ve modernized the words to make it a bit easier to read.
“Join the fellowship and communion with the saints in your area. It is no surprise to hear that a house gets robbed when it has no other houses around it. He that walks in communion of saints travels in company, he dwells in a city where one house keeps watch over another. The devil knows the damage he does in hindering this great ordinance of communion of saints – in doing this he hinders the progress of grace, indeed, brings that grace which Christians have into a declining, wasting state. The apostle couples these two duties close together, to ‘hold fast’ our ‘profession,’ and to ‘consider one another, to provoke unto love and to good works’ (Heb. 10:23-24).”
“Indeed, it is a dangerous step towards apostasy to forsake the communion of saints; so it is said of Demas that he ‘hath left us, and embraced the present world’ (2 Tim. 4:10).”
There are many good illustrations that show that Christian fellowship is good for our faith, spiritual strength, obedience, and sanctification. A soldier is weak apart from his unit, a coal gives off little heat alone, the ship that strays from the convoy is often sunk, predators go after the prey who has left the flock, and so on.
In another place Gurnall summarizes it well.
“God so orders things that we should need one another.”