Q. 54. What believest thou concerning the “holy catholic church” of Christ?
That the Son of God from the beginning to the end of the world, gathers,  defends, and preserves to Himself by His Spirit and Word, out of the whole human race, a church chosen to everlasting life, agreeing in true faith; and that I am and for ever shall remain, a living member thereof.
 John 10:11; Genesis 26:4;  Romans 9:24; Ephesians 1:10;  John 10:16;  Isaiah 59:21;  Deuteronomy 10:14–15;  Acts 13:48;  1 Corinthians 1:8–9; Romans 8:35, &c.
Q. 55. What do you understand by “the communion of saints”?
First, that all and every one, who believes, being members of Christ, are in common, partakers of Him, and of all His riches and gifts; secondly, that every one must know it to be his duty, readily and cheerfully to employ his gifts, for the advantage and salvation of other members.
 John 1:3–4; Romans 8:32; 1 Corinthians 12:13;  1 Corinthians 13:5; Philippians 2:4–6.
Q. 56. What believest thou concerning “the forgiveness of sins”?
That God, for the sake of Christ’s satisfaction, will no more remember my sins, neither my corrupt nature, against which I have to struggle all my life long; but will graciously impute to me the righteousness of Christ, that I may never be condemned before the tribunal of God.
 1 John 2:2; 2 Corinthians 5:19, 21;  Jeremiah 31:34; Psalm 103:3–4, 10–11; Romans 8:1–3;  John 3:18.
Some years ago I was at a meeting in which a Reformed minister was giving an introduction to the Creeds and Confessions of the Church. An elder from a certain Presbyterian church happened to be visiting, and when he heard the reference to the “holy catholic church” in the Apostles’ Creed, he was somewhat taken aback and asked why we were studying a Catholic creed! The minister could hardly believe his ear that the question was posed. But it did show how necessary his introduction was.
Well, the fact is that the term “catholic” does not refer to the “Roman Catholic Church.” It simply means “universal.” The “holy catholic church” refers to the Church universal. Now, traditionally Reformed theologians speak about the Visible Church Universal and the Invisible Church Universal. The Visible Church Universal comprises all the baptised members in congregations that may be regarded as true local churches. The Invisible Church Universal comprises all the elect of God throughout the world and throughout the ages, including those not yet born. A person who professes faith is a member of the visible church, although his membership is ratified only by water-baptism. On the other hand, a person who is an elect is a member of the invisible Church, and his membership is ratified by Spirit-baptism. Thus, we may say that the ratified membership of the Church Invisible is united by the indwelling of the Spirit of Christ rather than by membership in the local church.
By the term “holy catholic church,” the Apostles’ Creed and the Heidelberg Catechism are referring to the Church Invisible. But no member of the Church Invisible, whose membership has been ratified in regeneration, will remain outside the Church Visible for long. Their regeneration will always be followed by conversion and the experience of forgiveness of sin. Then they will inevitably seek membership in a local congregation so that they may not only enjoy the fellowship of other redeemed saints, but have the opportunity to exercise their spiritual gifts for the advantage and salvation of other Christians. This privilege of involvement in the lives and testimony of other believers is commonly denoted “communion of saints.”
Heidelberg Catechism >