Belief in the gospel, repentance from sin, and baptism (by full immersion) into Christ, are all part of the process of salvation: Matthew 28:19, Mark 16:16, John 3:23, Acts 2:38; 10:47; 16:31-33; 22:16; Romans 6:1-6, Galatians 3:27-29, Colossians 2:12, Ephesians 4:5, 1 Peter 3:21.
We are called on to be baptized as a public demonstration of our repentance: Acts 2:38; 22:16, 1 Peter 3:21. Since baptism is an outward reflection of our inner faith, it is right for us to obey the commandment to be baptized, and would be wrong for us to refuse baptism if it were possible: Acts 22:16.
However, there will certainly be people in the Kingdom of God who were not baptized simply because it wasn’t possible for them. This does not excuse from baptism those who knew they should be baptized, had the opportunity to be baptized, and refused baptism: Acts 22:16, James 4:17.
Also see:
Was the thief on the cross baptised?