The apostle John bears witness that blood and water flowed from Jesus' side (Jn 19:34). In 1 John 5:6, he explains that Jesus did not come by water only (here the water refers to baptism; notice water is mentioned first), but by water and blood, and that it is the Spirit who testifies. In other words, the effect of Jesus' blood, by the testimony of the Holy Spirit, is present during baptism to cleanse the sin of the person being baptized. Although the candidate's body is immersed physically in water, yet the blood of Jesus Christ cleanses his spiritual being (Heb 10:22; "washed" = immersed).
The baptisms performed in the True Jesus Church, during which many have witnessed blood in the water, confirm John's explanation.