“Why is baptism (immersion in water) essential for salvation?”
The short answer is, “Because God said so.” And, following whatever further discussion ensues, that is still the essence of any longer answer as well.
Why was it necessary for Naaman to dip seven times in the Jordan River to be healed of his leprosy (2 Kings 5.1-14)? Because God, through His prophet’s messenger, told him to do this.
Why was it necessary for the Israelites to march around the walls of Jericho for seven days, blow trumpets and shout (Joshua 6.1-27), even though from man’s standpoint that is an absurd way to capture a walled city? Because that’s what God said to do. “By faith the walls of Jericho fell down after they were encircled for seven days” (Hebrews 11.30).
Faith is the key.
Naaman didn’t deserve his healing because of his dipping in the Jordan River, but his dipping showed his faith and God saw his faith and healed him by grace.
The Israelites didn’t earn the city of Jericho by marching and shouting. “The LORD said to Joshua: ‘See! I have given Jericho into your hand, it king, and its mighty men of valor. You shall march around the city…’” (Joshua 6.2-3).
God gave them the city. They marched, as God told them to, and by marching they showed their faith. The city fell by grace through faith, but only after they marched as God told them to do.
Jesus has said, “He who believes and is baptized will be saved. He who does not believe will be condemned” (Mark 16.16). When one hears the gospel, believes and is baptized, that does not earn salvation. Salvation is a gift of God. Being baptized shows that the person believes what God says, “He who believes and is baptized will be saved.” Refusing to be baptized shows disbelief, “and he who does not believe will be condemned.”
Salvation is not by works of merit, not by works that would enable us to boast (Ephesians 2.8-9). Baptism is distinguished from that kind of work. God saves us, but “not by works of righteousness which we have done,” but “through the washing of regeneration” (Titus 3.4-6).
Baptism shows our faith (James 2.18), completes our faith (James 2.22), and makes our faith a living, active, effective faith instead of a dead, vain faith (James 2.20).
So we have come full circle. The answer is, “Because God said so.” He promises; we believe. He commands; we show our faith by obedience. When we have obeyed, God gives what He promised.
It is necessary to be immersed in water to be saved because that’s what God, who has salvation in His hand, says to do in order to receive it.
“Baptize” means immerse.
The word used in the original language means that. The English Bible also clearly shows that baptism is a burial (Romans 6.1-5; Colossians 2.12), and that in being baptized one goes down into and comes up out of the water (Acts 8.38).
Baptism is the point at which the believer comes into Christ (Galatians 3.27); has sins washed away (Acts 22.16); dies to sin and begins to walk in newness of life (Romans 6.1-4); receives remission of sins and the gift of the Holy Spirit (Acts 2.38); is saved (Mark 16.16; 1 Peter 3.21). We are commanded to be baptized with that in view:
“Repent and be baptized every one of you in the name of Jesus Christ for the forgiveness of your sins, and you will receive the gift of the Holy Spirit” (Acts 2.38).
“And now why do you wait? Rise and be baptized and wash away your sins, calling on His name” (Acts 22.16).
To be baptized believing one is already in Christ, has sins already washed away, is already dead to sin and walking in newness of life, has already received remission of sins and the gift of the Holy Spirit, and therefore, is already saved, is to deny every biblical purpose for baptism.
It’s important to note that baptism is not “a work done by us in righteousness” (Titus 3.5); it’s not a deed which merits anything from God. Baptism is in water, but the power is neither in the act nor in the water, but “through the resurrection of Jesus Christ” (1 Peter 3.21).
– Cecil May, Bible Questions and Answers, 17-20