The first testimonies are indicated by the Apostle in the three most important sacred events: the Lord's Baptism, His Passion, and the descent of the Holy Spirit upon the Apostles.
In these events the chief elements had great significance: water, blood, and fire. For this reason the Apostle calls these testimonies earthly.
The second triad of witnesses is heavenly: the Father, the Word, and the Holy Spirit. Their testimony was clearly expressed in the three most important events of the divine economy of our salvation.
These events were accompanied by extraordinary manifestations and divine signs. At the Baptism of Jesus Christ, the voice of God the Father was heard: "This is My beloved Son" (Mt 3:17; Mk 9:7).
The Spirit of God appeared there in the form of a dove. During Christ's sufferings, the saving Blood was shed, cleansing sinners and clearly testifying that the Sufferer was not a phantom.
In the descent of the Holy Spirit, sent by the Son of God from the Father, the wondrous gift of tongues was given to believers.
Many doubts are expressed against the authenticity of verse 7, and especially against our reading of its ending. This is because the verse is absent from some manuscripts of this Epistle.
For this reason some theologians do not cite it in confirmation of the doctrine of the Trinity. Yet the force of John's proofs concerning the divine dignity of Jesus Christ remains the same even without verse 7.
For he speaks of the testimony of the Spirit in verse 6, and of the testimony of the Father in verse 9.
Moreover, the omission of verse 7 in only some texts is not proof that it is inauthentic. One may think that the ending of verse 7, identical with the ending of verse 8, gave certain early copyists an easy opportunity for an accidental omission.
These faulty copies then served as models for those manuscripts in which verse 7 is absent.