“He who believes and is baptized will be saved; but he who does not believe will be condemned.” (Mark 16:16)
First, and this is an important preliminary statement, this portion of Mark 16, particularly verses 9 through 20, is not found in the earliest manuscripts we have of Mark’s Gospel. So, if we were teaching through this text verse-by-verse I would spend an extended amount of time dealing with the textual issue we’re presented with here, as well as the likelihood that this section was not in Mark’s original writing. But putting that aside, you can see that within the verse itself it is not the absence of baptism that condemns, it is the absence of faith – “he who does not believe will be condemned” (vs.16b). This would make sense seeing the abundant scriptural testimony of salvation by grace through faith.
How, then, should we understand the first half of the verse? Well, saying, “He who believes and is baptized will be saved” is like saying, ‘He who believes and walks in love will be saved’, or ‘He who believes and walks in the light will be saved.’ They are simply declarative statements that cohesively tie together Biblical truths (see Eph. 2:8-9; 1 Jn. 1:6-7; 4;8 for a series of Scriptural examples that exemplify the previous quotations). Faith alone saves, but the faith that saves is not alone, it comes with obedience (1 Jn. 2:3, i.e. baptism), love (1 Jn. 3:10b), and new desires for the things of God (1 Jn. 2:29; Ezek. 36:26).
Therefore, leaving aside the fact that Mark 16:9-20 is not found in the earliest manuscripts of Mark’s Gospel, Mark 16:16 should not be seen as teaching salvation by baptism; rather, if it were a part of Mark’s original autograph, it could be used as a proof text to show that it is not the absence of baptism that condemns but the absence of faith.