In the Bible, baptism involves the complete immersion of the believer in water. The word baptise comes from the Greek 'baptiso' which means to dip, immerse or submerge. The word implies change as it was associated with the dyeing trade where cloth was fully immersed in the dye to bring about a change in colour.
In acknowledgment of God's righteousness as a basis for the forgiveness of sins, the believer identifies with the death and resurrection of Christ through baptism. Under the water, the body 'dies' to serving sin and is then 'raised' to a newness of life as a servant of God.
The condition of 'belief' being stated means the believer has to understand the Gospel and respond accordingly.
“Now as they went down the road, they came to some water. And the eunuch said, ‘See, here is water. What hinders me from being baptised?’ Then Philip said, ‘If you believe with all your heart, you may.’ And he answered and said, ‘I believe that Jesus Christ is the Son of God.’ So he commanded the chariot to stand still. And both Philip and the eunuch went down into the water, and he baptised him” (Acts 8:36-38).
“. . . do you not know that as many of us as were baptised into Christ Jesus were baptised into his death? Therefore we were buried with him through baptism into death, that just as Christ was raised from the dead by the glory of the Father, even so we also should walk in newness of life. For if we have been united together in the likeness of his death, certainly we also shall be in the likeness of his resurrection, knowing this, that our old man was crucified with him, that the body of sin might be done away with, that we should no longer be slaves of sin. For he who has died has been freed from sin. Now if we died with Christ, we believe that we shall also live with him, knowing that Christ, having been raised from the dead, dies no more. Death no longer has dominion over him” (Romans 6:3-9).
(Quotations are from the New King James Version of the Bible)