What do you mean when you say, "I believe in the communion of the saints?"

The Westminster Confession of Faith (26.1-2) answers that question in a very helpful manner that shows how the good news of being made a part of this body leads us to respond by caring for the same:

All Saints, that are united to Jesus Christ, their head, by his Spirit and by faith, and have fellowship with him in his grace, suffering, death, resurrection, and glory. United to one another in love, God's people have fellowship in each other's gifts and grace and are obliged to perform those public and private duties which nourish their mutual good, both spiritually and physically.

By their profession of faith God's people are bound to maintain a holy fellowship and communion with each other in the worship of God and in the performance of other spiritual services for their mutual edification. They are also bound to help each other in material things according to their different abilities and needs. This fellowship is to be offered, as God gives the opportunity, to everyone in every place who calls on the name of the Lord Jesus.