In the Apostles' creed and the Westminster confession of faith we affirm the our belief in the communion of saints.
Westminster confession of faith, Chapter 26, section 1 & 2:
I. All saints that are united to Jesus Christ their head, by his Spirit and
by faith, have fellowship with him in his graces, sufferings, death,
resurrection, and glory: and, being united to one another in love, they have
communion in each other's gifts and graces, and are obliged to the performance
of such duties, public and private, as to conduce to their mutual good, both in
the inward and outward man.
II. Saints by profession, are bound to maintain an holy fellowship and
communion in the worship of God, and in performing such other spiritual services
as tend to their mutual edification; as also in relieving each other in outward
things, according to their several abilities and necessities. Which communion,
as God offereth opportunity, is to be extended unto all those who, in every
place, call upon the name of the Lord Jesus.
In his book "the Reformed faith, and exposition of the Westminster confession of faith", Robert Shaw helpfully writes:
"Communion is founded in union. The above sections embrace–First, The
union of the saints to Jesus Christ, and their communion with him; Secondly,
The union and communion of real saints with one another; Thirdly,
The union of saints by profession, and the communion which they are bound
Another reminder that we are not lone ranger Christians existing independently of the Church. Thank God for the communion of the saints! You can read the rest of Shaw's exposition here: