Got it. Anything in the Holy Bible to back all that up? I mean, to be convincing to a non-RC, wouldn't it be good to hear such an important part of theology from the lips of the Lord Jesus Christ Himself?
Sure.
The doctrine of the communion of saints was taught by the apostles, both in the Scriptures and the Tradition they handed down in words and practice. It is explicitly contained in the Apostles’ Creed. The Church reaffirmed this teaching at the Second Council of Nicea (787) and further addressed it at the Councils of Florence (1438–45), Trent (1545–63), and Vatican II (1962–65).
This communion refers to the bond of unity among the followers of Christ. Such a bond is possible because, as believers in Christ, we become children of God (1 Jn. 3:1), members of His family (Rom. 8:14–17), with divine life bestowed on us through Baptism (Jn. 3:3–5). The apostles teach us that through Baptism we become “fellow heirs with Christ” (Rom. 8:17) and “partakers of the divine nature” (2 Pet. 1:4). Saint Paul states that this union of the faithful, brought about by the Holy Spirit in Baptism, is so complete that we are actually members of a single body, Christ’s own body (cf. 1 Cor. 12:12–27).
The communion of saints is based on four essential points:
All Christians are members of Christ’s body and one another (Rom. 12:5; 1 Cor. 12:27).
Jesus has only one body (Eph. 4:4; Col. 3:15).
Death cannot separate Christians from Christ or from one another (Rom. 8:35–39; cf. Lk. 20:37–38).
Christians are bound in mutual love (Jn. 13:34–35; Rom. 12:10).
Suprenant, L. J., Jr., & Gray, P. C. L. (1999). Faith Facts: Answers to Catholic Questions (Vol. 1, pp. 33–34). Steubenville, OH: Emmaus Road Publishing.
Please show the scripture that shows the apostles teaching to pray to or through those who have passed from this earthly life.