The three who testify
Whether the heavenly witnesses belong to the text or entered it late.
1 John 5:7-8
7For there are three who testify:
8the Spirit, the water, and the blood; and the three agree as one.
What each tradition says
Rabbinic / Midrashic
No sourced position found. Checked the Jewish Encyclopedia (1905): arts. "Trinity" (id 14519), "Christianity in Its Relation to Judaism" (4366), "Son of God" (13912), "Logos" (10090 - a cross-reference stub), "Troki" (14525 - summarizes Hizzuk Emunah's Messiah arguments only); no statement on the authenticity of 1 John v. 7 found.
Patristic
Cyprian cites what is written concerning the Father, the Son, and the Holy Spirit: “And these three are one.” He presents this as written testimony that these three are one.
Cyprian of Carthage, On the Unity of the Church (ANF vol. 5) — § 6 - on the Lord's prayer that the Church be one
Catholic
Haydock notes that Socinians object this verse is absent from many Greek manuscripts, and that Erasmus in one edition and Mr. Simon questioned or rejected it as a false reading. He says this was done without sufficient proofs and grounds, as learned Catholics and Protestant writers have shown, and those Protestants receive the verse as canonical in their translations.
George Leo Haydock, George Haydock's Catholic Bible Commentary (1859) — Comment on 1 John v. 7
Orthodox
No sourced position found. Checked the Synod of Jerusalem (Confession of Dositheus, J. N. W. B. Robertson trans., 1899, archive.org actsdecreesofsyn00orth), the Greek fathers in ANF/NPNF (Chrysostom, Cyril of Jerusalem, John of Damascus), and Philaret's Longer Catechism - no Eastern Orthodox statement on the authenticity of 1 John v. 7 found.
Reformed / Protestant
Calvin notes some omit this whole verse; Jerome thought this happened by design, only among the Latins, not by mistake. Because the Greek copies disagree, Calvin will not assert anything. Yet since the clause makes the passage flow better and is found in the best approved copies, he inclines to receive it as the true reading.
John Calvin, Commentaries on the Catholic Epistles (Calvin Translation Society) — Comment on 1 John v. 7
Evangelical
Wesley notes that 1 John 5:7, usually so reckoned, briefly recapitulates all previously advanced about the Father, the Son, and the Spirit. He adds that it is cited together with verses 6 and 8 by Tertullian, Cyprian, and an uninterrupted train of Fathers.
John Wesley, John Wesley's Notes on the Bible, Notes on the First Epistle of St. John — Note on 1 John v. 7 (in the note on v. 4-5)
Evangelical
Barnes says the disputed clause is missing from all earlier Greek manuscripts and found in no Greek manuscript before the sixteenth century. Only two Greek manuscripts of any age contain it: Codex Montfortianus and Codex Ravianus, which he calls a mere transcript. It is incredible that a genuine New Testament passage would be missing from all early Greek manuscripts.
Albert Barnes, Albert Barnes' Bible Commentary (Notes on the New Testament) — Note on 1 John v. 7, first reason against the disputed clause