If A then B does not imply If not A then not B(inverse). It does imply If not B then not A(contrapositive).
If you believe and are baptized then you are saved Does not imply If you do not believe and are not baptized then you are not saved.
unless A then not B is linguistically equivalent to if -A then -B .
if B then A is logically implied.(contrapositive)
unless you repent then you are not saved.
is equivalent to
If you do not repent then you are not saved
Which implies
If you are saved then you repented.
and in context it reads
12After that he appeared in another form unto two of them, as they walked, and went into the country.God has clearly said -- believe and be baptise and you shall be saved. If you don't believe, then baptism can't help you and you shall be damned.
13And they went and told it unto the residue: neither believed they them.
14Afterward he appeared unto the eleven as they sat at meat, and upbraided them with their unbelief and hardness of heart, because they believed not them which had seen him after he was risen.
15And he said unto them, Go ye into all the world, and preach the gospel to every creature.
16He that believeth and is baptized shall be saved; but he that believeth not shall be damned.
But the clear point is you believe then you get baptised.
These are the words of Christ.
Baptism sans belief does not help, but Christ says believe and be baptised -- it is if you do A (believe) then get B (baptism) done. If you don't do A, then B has no point. This does not mean that B is not necessary, just that it does not stand on its own.
For the Lord says, ‘Except a man be baptized of water and of the Spirit, he shall by no means enter into the kingdom of heaven.’