Having just summed up all three tests for authentic Christianity (5:1-5), John emphasizes the doctrinal test again. As he moves toward the end of his letter, John stresses belief in Jesus because he wants to show the priority of belief in Jesus as God’s full and final testimony about Himself.

Verse 6
By mentioning Jesus’ coming by "water and by "blood," he draws his readers’ attention to the historical, empirical, and verifiable evidence of Jesus’ baptism and crucifixion.

The "one" baptized and crucified is "Jesus Christ." Cerinthus, one of John’s contemporaries, taught that the Christ came upon Jesus at baptism then left him prior to the crucifixion. John insists that "He did not come by water only, but by water and blood."

Jesus was and remains the Christ of God. It was not merely the human Jesus but the divine Christ who made atonement for sin on the cross. We must never demote Jesus Christ to a mere man. Otherwise, we lose the principal doctrine of salvation, namely that "God was in Christ reconciling the world to himself" (2 Cor. 5:19).

As a wise pastor, John anticipates that his readers will not rely solely on his human testimony, but points them to the Spirit’s testimony about Jesus. Although he points to the empirical evidences about Jesus, John depends heavily upon God’s Spirit to take the available evidence of Jesus’ identity and embed it into the hearts of God’s people (see 2:20, 27, 4:2, 13).

The Spirit, Who is the truth, speaks only the truth and all the truth about Jesus (15:26; 16:13-15).

Verses 7-8
The reading of the KJV (marginalized in the NASB, NIV, and NRSV) is a latter addition and has no textual basis. It did not appear in any Greek manuscript until 1215 a.d.

The Holy Spirit does not operate in a vacuum; He works in, with, and through reality (cf., Jn. 14:26). Note: The Spirit is not an impersonal force nor influence. He "testifies" and is in "agreement" with the testimony about Jesus.

The operation of God’s Spirit always operates through both objective and subjective reality (see Jn. 16:8-11 for an example of the latter). Undeniably, Jesus Christ was baptized and crucified. Baptism was the inauguration of Jesus’ ministry (Mk. 1:11; Jn. 1:32-34 where Jesus was declared divine, not made divine). Crucifixion was the culmination of His ministry (Acts 2:36; 1 Cor. 1:23; 2:2; Gal. 2:20; Gal. 3:1 where it is "Christ" Who was crucified).

All three evidences about Jesus stand or fall together. One could not accept the Spirit’s testimony and reject the evidence of the baptism and crucifixion.

There is no reason to believe that John is taking a leap to discuss Christian Baptism or the Lord’s Supper, as some have suggested.

Verse 9
John begins with the empirical and advances to the theological evidences. Note the progression of testimony. First, it is the testimony of Jesus’ baptism and crucifixion. Second, the Spirit testifies through Jesus’ baptism and crucifixion. Now, it is God’s testimony.

To Consider: External signs often lead to the internal witness of the Spirit Who complements the obvious. Though Christian faith is subjective in character, it is rooted in the objective.

The reason why the three agree is because God is behind their testimony. John’s concern is to show the continuity and integrity of God’s self-revelation in Christ. Since God cannot deny himself (2 Tim. 2:13), nor lie (Num. 23:19; Heb. 6:18), it is the height of foolishness not to take Him at his word concerning the identity of Jesus.

Verse 10
Having God’s testimony about Jesus in one’s heart is both the cause and consequence of true faith. Human acceptance leads to divine confirmation.

Observe the sequence: Faith precedes assurance and certainty. Moreover, faith is not irrational nor is it a blind leap. Faith is "resting in the evidence" (Augustine).

The effect of unbelief is contradicting the Almighty God of the universe; the ultimate blasphemy! Unfortunately, many hold to belief in God, but deny His testimony about Jesus. According to John, this is entirely incongruous. It is impossible to maintain belief in God while rejecting His revelation about Himself (see Jn. 3:32-33).

Summary
For John, and for all believers, there can be no middle ground about the revelation of God in Christ. Within verses 6-11 the word for "testimony" (NIV) or "witness" (NASB) occurs 6 times as a noun and 4 times as a verb. God has fully and finally revealed Himself in His Son (Heb. 1:1-2). This testimony is historically reliable, logically consistent, and completely accessible through the witness of the Spirit.

 1 John 5:11-21