Jamieson, Fausset and Brown on Mark 16:16

16. He that believeth and is baptized--Baptism is here put for the external signature of the inner faith of the heart, just as "confessing with the mouth" is in Ro 10:10; and there also as here this outward manifestation, once mentioned as the proper fruit of faith, is not repeated in what follows (Ro 10:11).

shall be saved; but he that believeth not shall be damned--These awful issues of the reception or rejection of the Gospel, though often recorded in other connections, are given in this connection only by Mark.