What does Mark 16:16 mean?

The wording found in this verse is unfortunate for two reasons. First and foremost, the entire section of Mark 16:9–20 is not found in the earliest, most reliable manuscripts. Most Bible scholars do not think this was a part of the original, divinely-inspired writing of Mark's Gospel. Secondly, this passage is all-too-easily misinterpreted to imply that baptism is required for salvation.

Logically, however, even if accepted as original to Mark, these words do not prove baptism is a requirement for eternal life. Other passages affirm that belief alone is necessary for salvation (John 3:18; 5:24; 12:44–50; 20:31; Romans 10:9; 1 John 5:13). Even the last half of this verse states that lack of belief is all that is necessary to not be saved.

Baptism is found in proximity with belief in Christ in the New Testament because of the culture. In the time of the early church, if someone joined a particular sect of Judaism they would publicly declare their allegiance with baptism. This is why John the Baptist baptized his followers (John 1:26–28). In those times, a new believer didn't have to go through a class or give a detailed testimony in order to be baptized. They were baptized as soon as they affirmed the teacher's message, like the Ethiopian court official who met Philip (Acts 8:26–40). Consider also the thief on the cross who declared his faith in Jesus (Luke 23:39–43). Jesus assured him he would go to paradise with no mention of baptism.

Baptism is not a requirement for salvation, it's a response to salvation. When we are baptized, we are giving a public affirmation that we choose to follow Jesus. The Bible talks about three different baptisms. "John's baptism" was for those before the resurrection who agreed that their sin was wrong and they resolved to live a better life (Matthew 3:5–6, 11; Acts 19:4). Believers' baptism is for those who understand that while repentance is important, we can never be so good that we can earn salvation; it is performed after we are saved. The baptism of the Holy Spirit is what we receive at the moment of salvation. The Holy Spirit dwells within us permanently, marking us as saved by Christ (Mark 1:8; 1 Corinthians 12:12–13; Ephesians 1:13–14).