And there are three that bear witness in earth, the spirit, and the water, and the blood: and these three agree in one. The Spirit, and the water, and the blood - This verse is supposed to mean "the Spirit - in the word confirmed by miracles; the water - in baptism, wherein we are dedicated to the Son, (with the Father and the Holy Spirit), typifying his spotless purity, and the inward purifying of our nature; and the blood - represented in the Lord's Supper, and applied to the consciences of believers: and all these harmoniously agree in the same testimony, that Jesus Christ is the Divine, the complete, the only Savior of the world." - Mr. Wesley's notes.
By the written word, which proceeded from the Holy Spirit, that Spirit is continually witnessing upon earth, that God hath given unto us eternal life.
By baptism, which points out our regeneration, and the renewing of the Holy Ghost, and which is still maintained as an initiatory rite in the Christian Church, we have another witness on earth of the truth, certainty, importance, and efficacy of the Christian religion. The same may be said of the blood, represented by the holy eucharist, which continues to show forth the death and atoning sacrifice of the Son of God till he comes. See the note on 1 John 5:6.