What is the status of someone who repents of their sin and professes their belief in Christ, but then decides not to be baptized?
If a person meets the test of Romans 10:9, then the promise of scripture is they are saved:
Rom. 10:9 that if you confess with your mouth Jesus as Lord, and believe in your heart that God raised Him from the dead, you will be saved.
Therefore, a refusal to be water baptized does not - by itself - prove a lack of salvation.
Baptism is an important step of obedience for a Christian, but water baptism is not a required for salvation. We strongly believe that each and every Christian should be water baptized by immersion. Baptism illustrates a believer’s identification with Christ’s death, burial, and resurrection. Romans 6:3-4 declares, “Or don't you know that all of us who were baptized into Christ Jesus were baptized into His death? We were therefore buried with him through baptism into death in order that, just as Christ was raised from the dead through the glory of the Father, we too may live a new life.” The action of being immersed in the water illustrates dying and being buried with Christ. The action of coming out of the water pictures Christ’s resurrection.
Nevertheless, the only baptism required for salvation is the baptism of the Holy Spirit, which accompanies a true confession of faith. Requiring anything in addition to faith in Jesus Christ for salvation is a works-based salvation and is false. If a believer refuses water baptism, then they are disobeying the Lord and will not find themselves growing in His grace so long as they are determined to disobey His word.
A refusal to accept water baptism might be an indication that the person's confession of faith was not sincere. In such a case, we could say that their refusal is the evidence of unbelief, but since we cannot know their heart with certainty, we cannot construct a general rule from this situation. A refusal to be water baptized may be a sign of unbelief, but it can also be evidence of a believer's disobedience or spiritual immaturity. Both situations are possible, so we cannot make a rule.
If you are interested in reading more about the difference between water baptism and the baptism of the Holy Spirit, please read the following article: Is Baptism Required for Salvation.