"He that believeth and is baptized shall be saved; but he that believeth not shall be damned." This verse from Mark shows 2 things necessary to be saved. Most people can understand what is meant by the term "believeth", which is the first thing. Of course, "believeth" means to believe on the Lord Jesus for your salvation. The other thing necessary for salvation, according to this verse from Mark, is to be baptized.
But those who still accept v.15 need to realize that what will damn a person is those who "believeth not". That is exactly what the verse says. It leaves OUT any mention of baptism.
And this parallels two other verses about who will be condemned.
John 3:18 - Whoever believes in him is not condemned, but whoever does not believe stands condemned already because they have not believed in the name of God’s one and only Son.
2 Thess 2:12 - and so that all will be condemned who have not believed the truth but have delighted in wickedness.
Couldn't be more clear. Condemnation is for those "who have not believed" as both verses state plainly. And agrees with Mark 16:15.
What being baptized means, however, is subject to interpretation. If we are not paying attention to how God tells us to interpret the Bible (in Psalm 78:1-2, and in Mark 4:34), we might interpret 'baptism' to mean being washed or dunked in physical water in Jesus' name. However, this seems a little bit amazing. Can physical water really contribute to one's salvation? Sure it could wash off physical dirt but what about the stain of sin? We read about the stain of sin being washed away. It is in the Bible: "That he might sanctify and cleanse it with the washing of water by the word". That is interesting. Jesus, who is the word of God, can cleanse or wash a believer so that they are free from sin and sanctified to God. The word 'baptize' means 'to wash'. Look it up in a Concordance if you need to. So, when we read in Mark 16:16: "He that believeth and is baptized shall be saved; but he that believeth not shall be damned" , we can understand that it means whoever believes on the Lord Jesus Christ unto salvation and who also is washed free from the stain of sin (that means forgiven) will be saved.