Question: "What does 'communion of the saints' mean in the ancient creeds?"
Answer: The word communion means “an act or instance of sharing.” When the Apostles’ Creed speaks of the “communion of the saints,” it is not just specifically referring to the Lord's Supper/Holy Communion in which believers come together to remember Jesus’ sacrifice through the bread and the wine. The communion of the saints is much deeper than this important ordinance.
The communion of the saints refers to believers in the past, believers in the present, and believers in the years to come sharing a common salvation in our Savior the Lord Jesus Christ. Those who have passed from this world into the presence of Christ at the death of their body have the same salvation as believers who are alive today. Those who will come after us will have the same salvation we have.
The communion of saints is all what each one of us has in common with all other believers. We have been forgiven through His death and saved by His life (Romans 5:10), set free from the law of sin and death (Romans 8:2), and passed from spiritual death to eternal life (1 John 3:16). We have all been permanently adopted as children of God by the will of God (John 1:12-13). We have become God's handiwork to do good works that He has prepared for us (Ephesians 2:10), and we have all been given an eternal home in the New Jerusalem in the presence of our Lord Jesus Christ, our Lamb, our Temple, and our Light (Revelation 21:22–27). These are only a few of the blessings we have in common with all other believers.