1 John 5:8

And there are three that bear witness in earth, the Spirit, and the water, and the blood; and these three agree in one.

St. John here shows that the foundation of our faith is absolutely firm and certain, since it rests upon the powerful testimony of God Himself: This is He that came through water and blood, Jesus Christ; not in water alone, but in water and in blood; and the Spirit is He that testifies, because the Spirit is the Truth. Here the two principal events in the life of Jesus are set before our eyes, namely, His baptism, by which He entered upon His public ministry, and His suffering and death, through which He crowned His work of redemption. These two events prove with special force that Jesus is the Christ, the Savior of the world. He accepted the baptism intended for sinners and thereby declared His willingness to make full satisfaction for the sins of the world. He shed His blood and gave His life into death for the reconciliation of the world. And it was not only His first willingness to undertake the work of salvation which counted, but the shedding of His blood, His suffering and death. Of these facts the Spirit of God in the Gospel bears witness, testifying without ceasing that Jesus Christ is the Savior of the world. That is the special work of the Holy Ghost, to testify regarding the truth, to teach the truth, since He Himself is the Truth, the eternally faithful God. Thus the testimony of the Spirit glorifies Christ in the hearts of the believers.

The text continues: For three there are that bear witness in heaven, the Father, the Word, and the Holy Ghost; and these three are one. This is the great mystery of the Trinity: God the Father, God the Son, the eternal Word, and the Holy Ghost, three in persons, one in essence. These three in one testify in behalf of Jesus that He is the Christ, the Savior of the world. And with their testimony agrees that of three witnesses on earth: the Spirit and the water and the blood; and these three concur in one. Here on earth the Holy Ghost is the chief witness. As He led the disciples of Christ into all truth and inspired them to write the Gospel of Jesus Christ, the Savior of the world, so He still works faith in our hearts through the Word of the Gospel, He still teaches us the value of the other witnesses for Christ's redemption, of His baptism and of His suffering and death. Thus we have unmistakable and incontrovertible evidence for the fact that Jesus, our Savior, really completed the work of redemption, gained a perfect atonement for the whole world. Thus the three witnesses have only one object, namely, to point to Christ, to testify to the salvation which we have in Him.