Exodus 12:21+ Moses called all the elders of Israel and said to them, "Go and procure lambs for your families, and slaughter them as Passover victims. Then take a bunch of hyssop, and dipping it in the blood that is in the basin, sprinkle the lintel and the two doorposts with this blood. But none of you shall go outdoors until morning. For the LORD will go by, striking down the Egyptians. Seeing the blood on the lintel and the two doorposts, the LORD will pass over that door and not let the destroyer come into your houses to strike you down. You shall observe this as a perpetual ordinance for yourselves and your descendants.
Ritual sprinkling of blood on the faithful confirms their inclusion in the Passover Covenant:
And Moses....rose up early in the morning, and built an altar.....and sacrificed peace offerings of oxen unto the LORD. And Moses took half of the blood, and put it in basins; and half of the blood he sprinkled on the altar. And Moses took the blood, and sprinkled it on the people, and said, Behold the blood of the covenant, which the LORD hath made with you......
Leviticus 8:30 And Moses took some of the anointing oil and the blood which was upon the altar, and sprinkled it upon Aaron, and upon his garments, and upon his sons, and upon his sons' garments with him; and sanctified Aaron, and his garments, and his sons, and his sons' garments with him.
Blood and water together are sprinkled as a part of a ritual cleansing process:
Leviticus 14:2+ This shall be the law of the leper in the day of his cleansing ...the priest [will take] two birds alive and clean, and cedar wood, and scarlet, and hyssop. And the priest shall command that one of the birds be killed in an earthen vessel over running water. As for the living bird, he shall take it, and the cedar wood, and the scarlet, and the hyssop, and shall dip them and the living bird in the blood of the bird that was killed over the running water. And he shall sprinkle upon him that is to be cleansed from the leprosy seven times, and shall pronounce him clean, and shall let the living bird loose into the open field.
Purification of the unclean requires the sprinkling of water made holy by the addition of the ashes of a sacrificial victim:
Numbers 19: 17-18 And for an unclean person they shall take of the ashes of the burnt heifer of purification for sin, and living (i.e. running) water shall be put in a vessel: And a clean person shall take hyssop, and dip it in the water, and sprinkle it upon the tent, and upon all the vessels, and upon the persons that were there, and upon him that touched a bone, or one slain, or one dead, or a grave.
Washing in water effects miraculous physical healing:
2 Kings 5: Now Naaman, captain of the host of the king of Syria, was...a mighty man in valor, but he was a leper. So Naaman came with his horses and with his chariot, and stood at the door of the house of Elisha. And Elisha sent a messenger unto him, saying, Go and wash in Jordan seven times.....and you will be clean.....Then went he down, and dipped (baptized in Greek) himself seven times in Jordan, according to the saying of the man of God: and his flesh came again like the flesh of a little child, and he was clean.
Ezekiel prophesied that sprinkled water would effect spiritual cleansing:
Ezekiel 36:24+ For I will take you from among the heathen, and gather you out of all countries, and will bring you into your own land. Then will I sprinkle clean water upon you, and ye shall be clean: from all your filthiness, and from all your idols, will I cleanse you. A new heart also will I give you, and a new spirit will I put within you: and I will take away the stony heart out of your flesh, and I will give you a heart of flesh.
John the Baptist's ministry included a symbolic water baptism of repentance:
Matthew 3:4+ ...Jerusalem and all Judea and all the region about the Jordan were going out to him, and they were baptized by him in the river Jordan, confessing their sins. "I baptize you with water for repentance, but he who is coming after me is mightier than I, whose sandals I am not worthy to carry. He will baptize you with the Holy Spirit and with fire...And Jesus, when he was baptized, went up straightway out of the water: and the heavens were opened unto him, and he saw the Spirit of God descending like a dove, and lighting upon him. (note that as in Numbers, the living water was made holy by the immersion of a sacrificial victim's ashes, as the water of Baptism was made holy by the immersion of Jesus, the perfect victim, into it.)
Blood and water together flowed out of Jesus' crucified body:
John 19:31+ Since it was the day of Preparation, and so that the bodies would not remain on the cross on the Sabbath (for that Sabbath was a high day), the Jews asked Pilate that their legs might be broken and that they might be taken away. So the soldiers came and broke the legs of the first, and of the other who had been crucified with him. But when they came to Jesus and saw that he was already dead, they did not break his legs. But one of the soldiers pierced his side with a spear, and at once there came out blood and water.
The water and Spirit, present when John baptized Jesus, are joined to the blood of Jesus, the sacrificial Lamb of the New Covenant:
1 John 5:6+ This is he who came by water and blood: Jesus Christ; not by the water only but by the water and the blood. And the Spirit is the one who testifies, because the Spirit is the truth. For there are three that testify: the Spirit and the water and the blood; and these three are of one accord.
Thus Trinitarian water Baptism includes the blood of Jesus' sacrifice. This enables us to be sprinkled in the sacrificial blood of the New Covenant, just as the Israelites were sprinkled with sacrificial blood of the Old Covenant. Catholics will recognize a parallel during the Rite of Blessing and Sprinkling Holy Water, in which holy water is sprinkled upon the entire congregation.
Christian Baptism for forgiveness of sins is likened to death and rebirth through Jesus' resurrection:
Colossians 2:12 having been buried with [Jesus] in baptism...you were also raised with him through faith in the powerful working of God, who raised him from the dead.
Revelation 7:13+ "And one of the ancients answered, and said to me: These that are clothed in white robes, who are they? and whence came they? 14 And I said to him: My Lord, thou knowest. And he said to me: These are they who are come out of great tribulation, and have washed their robes, and have made them white in the blood of the Lamb." That is, through water baptism the saved were washed in the blood of the sacrificial victim.
Christian Baptism for forgiveness of sin recalls the Old Covenant sprinkling of blood and sprinkling of water; Ezekiel's prophecy of sprinkling water for spiritual cleansing; immersion in water for miraculous healing; the blood and water of Jesus' sacrifice; and immersion in water symbolizing death by drowning, then rebirth. Baptism by immersion emphasizes some, but not all of these aspects.
The Didache, (Greek, "teaching") one of the oldest Christian documents, was written around 80 A.D, as were some books in the New Testament. It was intended for the instruction of converts to Christianity. It shows that Baptism by either immersion or pouring was considered valid by the first Christians.
Didache, part 2: "Concerning baptism, baptize in this manner: Having said all these things beforehand, baptize in the name of the Father and of the Son and of the Holy Spirit in living water [that is, in running water, as in a river]. If there is no living water, baptize in other water; and, if you are not able to use cold water, use warm. If you have neither, pour water three times upon the head in the name of the Father, Son, and Holy Spirit."
Christian denominations which only immerse will necessarily have a different understanding of these passages. Catholics believe the Church's understanding is authoritative.