What do you understand by the “communion of saints?”

Heidelberg Catechism

Question 55. What do you understand by the “communion of saints?”

Answer. First, that believers, one and all, as members of the Lord Jesus Christ, are partakers with Him in all His treasures and gifts;[1] second, that each one must feel himself bound to use his gifts readily and cheerfully for the advantage and welfare of other members.[2]

[1] Rom 8:32; 1 Cor 6:17, 12:4-7, 12-13; 1 Jn 1:3;
[2] Rom 12:4-8; 1 Cor 12:12-13, 20-27, 13:1-7; Php 2:4-8; Heb 3:14