And Jesus Christ was revealed as God’s Son by his baptism in water and by shedding his blood on the cross—not by water only, but by water and blood. And the Spirit, who is truth, confirms it with his testimony. So we have these three witnesses— the Spirit, the water, and the blood—and all three agree.
Testimony is a funny thing. Some people get weird ed out by having testimony in church, others probably are on the other end of the spectrum, with testimony about everything. Here John explains that Jesus is the Son of God, both divine and human. Salvation occurs in belief that Jesus was born of God, Baptized by John in water, and died a human death with Blood. The water and blood give us the testimony that Jesus was born of the spirit in water, and died a very human death on the cross, through his blood. So water and blood symbolize for John and his community a recognition of salvation coming through the Son of God, born of a virgin Mary, made human, and named God's beloved at his baptism. these two, water and blood, testify that Jesus was the Son of God, who takes away the sin of the world.
And since John is writing to those who did not see and touch Jesus, the Spirit confirms this truth for us who believe Jesus is the Son of God by their testimony. This testimony is greater than any human testimony.
Water and Blood are important aspects of our Christian identity. It is in these sacraments, Baptism and Holy Communion, that we recognize Jesus promise and presence. The Spirit's confirmation of these sacraments is added testimony. These are signs of God's grace active in the world.
Lord, remind us thru the water and the blood you extend grace upon grace, upon grace to us as signs of your love, confirmed by your Holy Spirit. - Amen