When it comes to baptism, Christians don’t always agree on everything. But most could agree with this: The external water of baptism does not wash away sin; only the blood of Jesus Christ and the Holy Spirit can do that. So then it could be asked, “What is the point of the water?”
Put simply, the water is an illustration.
Of course, baptism is much more than an illustration. It is an act of God whereby he makes bold promises of grace to his people. But baptism is also an illustration; a sign. This is why baptism takes place in public worship in front of the congregation, with “visible water” so that God’s lessons can be “set before our eyes to be looked upon.”  Through baptism God illustrates three critical lessons that we need to learn in order to live and die happily.
Baptism Illustrates Our Guilt
In the Old Testament, the “various washings”—literally “baptisms”—taught that sin makes people dirty in God’s sight (Heb. 9:10). Most of the baptism-washings in the Old Testament are described in Leviticus 15-17 in which the writer drives home his main point: People need to be cleansed from their sins! (see Lev. 16:16). At the close of the Old Testament age John preached a baptism of repentance for the remission of sins (Mark. 1:4). And after Christ’s death people like Paul were baptized because of their sins. Before his conversion, Paul was a “blasphemer, a persecutor, and an insolent man,” the chief of sinners (1 Tim. 1:13, 15). By nature and by his actual sins, Paul had alienated himself from the life of God. When God saved Paul, he told him to “be baptized, and wash away your sins” (Acts 22:16).
All of us need to be washed. Like King David in Psalm 51, our bothered consciences betray our guilt. David had slept with another man’s wife and killed that man to cover up his adultery (see 2 Samuel 11). And though he acted secretly (2 Sam. 12:12), his sin was always before him (Ps. 51:3).
Baptism teaches us, in the words of one historic form, to “loathe ourselves, humble ourselves before God, and seek for our purification and salvation apart from ourselves.” Augustus Toplady put it this way, “Nothing in my hand I bring, simply to thy cross I cling; naked, come to thee for dress; helpless, look to Thee for grace; foul, I to the fountain fly; wash me, Savior, or I die.
The water of baptism invites cleansing.
Baptism Illustrates God’s Grace
David could not wake up from the nightmare of his own sin until he turned to the Lord for cleansing. He prayed for a sort of baptismal washing. “Purge me with hyssop, and I shall be clean; wash me, and I shall be whiter than snow” (Ps. 51:7). Hyssop is a desert plant that was used to sprinkle liquid for ceremonial washings (see Num. 19:18). Sometimes hyssop was used to sprinkle blood; Moses “took the blood of calves and goats, with water, scarlet wool, and hyssop, and sprinkled… all the people… and according to the law almost all things are purified with blood, and without shedding of blood there is no remission” (Heb. 9:19, 22).
David cried out to God to baptize him, to truly purify him from sin. And not merely with water or the blood of animals sprinkled from the hyssop plant, but also with the water and blood that would be pressed out of the body of God’s Son.
Calvary’s cross was the hyssop that sprinkled upon God’s chosen ones the blood and water of Jesus Christ, which alone, received by faith, can wash away sin. Baptism teaches believers that as certainly as they are washed by water, so surely are they cleansed of all their sins by Christ’s blood and Spirit. When you come to Jesus dirty, guilty, undeserving of mercy, frustrated with yourself and your pathetic David-like catharsis, and ask God to wash you, “in the name of the Lord Jesus Christ and by the Spirit of our God,” and you will be washed (1 Cor. 6:11).
The water of baptism assures God’s people of their cleansing.
A Lesson about Gratitude
The person who truly knows what it means to be baptized by the blood of Christ is a thankful person. David sings a song of gratitude when he says, “Deliver me from the guilt of bloodshed, O God, the God of my salvation, and my tongue shall sing aloud of Your righteousness” (Ps. 51:14). Paul sings the song of gratitude when he says, though I am the chief of sinners, “I obtained mercy! …Now to the King eternal, immortal, invisible, to God who alone is wise, be honor and glory forever and ever, Amen” (1 Tim. 1:15-17).
Robert Robinson’s words resonate with baptized believers, “O to grace how great a debtor daily I’m constrained to be; let Thy goodness, like a fetter, bind my wandering heart to Thee.” God isn’t looking for followers who will pay him lip service. He doesn’t want people to serve him because they think it is their duty to do so. He is seeking spiritual worship from those who are deeply grateful that God has washed away their sins and has renewed them by his mercies (Rom. 12:1).
The water of baptism teaches believers to live the renewed life God has created for them. It revitalizes them with the message of God’s matchless love for his children. When believers feel ungrateful, unsettled, unhappy, unloved, or unimportant, their baptism reminds them: You are included in God’s covenant and his family. Christ has promised to you his blood and Spirit. God has inviolably pledged to be a God to you! (Gen. 17:7).
It turns out that water—one of earth’s most familiar substances—can be used by God to impress upon his people his amazing plan of salvation.
 Second Helvetic Confession 20.2.