Are you kidding me? There have been many people in various false religions and cults who have received water baptism because they have been taught by their church they they must be water baptized to receive salvation, yet they are not authentic believers. I've heard numerous testimonies from people who have received Christ through faith and are receiving water baptism, now as a believer, that admitted to receiving water baptism at their previous church, prior to truly receiving Christ through faith and were not genuine believers when they were previously baptized. I have that same testimony.
I've never heard of an atheist receiving water baptism, yet there is still a stage in the progress of belief in Jesus that "falls short of genuine or consummated belief resulting in salvation." Such people may believe "mental assent" in the existence and historical facts about Christ, such as He is the Son of God and His death, burial and resurrection "happened," yet they are still not trusting in the death, burial and resurrection of Christ as the ALL-sufficient means of their salvation and are instead, also trusting in works for salvation.
There is a need to mention, "whoever is not baptized will be condemned," if baptism was absolutely necessary for salvation.
Again, general cases without making a qualification for the unusual case of someone who believes but is not baptized, but he who does not believe will be condemned.
For the believer, Christ's finished work of redemption is sufficient and complete to save believers. No supplements needed. For the unbeliever, Christ's finished work of redemption is IN-sufficient to save, so they turn to supplements. The believer trusts in Christ alone for salvation and not in works.
Believers are commanded to be water baptized after conversion (Acts 10:43-48). I received water baptism after my conversion, but I was not saved by water baptism, but by believing in the Lord Jesus Christ (Acts 16:31).
Why would a believer refuse to be water baptized? I can't think of one believer that I know who has refused water baptism after their conversion. I don't teach believers to neglect getting water baptized, but I also don't teach baptismal regeneration.