Mark 16:15-16
15 And he said unto them, Go ye into all the world, and preach the gospel to every creature.
16 He that believeth and is baptized shall be saved; but he that believeth not shall be damned.
The Grace New Testament Commentary - Mark 16:16
16:16 The statement "He who believes and is baptized will be saved" is often cited to prove that water baptism is a condition of justification. However, there are several reasons why this is false. First, water baptism is not a condition for justification elsewhere in the NT (cf. John, Romans, Galatians). Second, the only condition of justification in Scripture is believing in Jesus (cf. Rom 3:1-8; Gal 2:16; 3:6-14). Baptism is the first step that one who has believed should take (Matt 28:19). The Lord concludes this statement by saying, "he who does not believe will be condemned." Thus the basis of condemnation is not unbelief plus failure to be baptized, but only unbelief.
Another view sees the words will be saved as referring to deliverance eternally as well as here and now. To escape eternal condemnation requires only belief in Christ (cf. Mark 16:16b), whereas to be saved from the wrath of God in this life requires belief and obedience (cf. Rom 10:9-14), symbolized by being baptized (Mark 16:16a).
Still another view sees this future salvation as referring to more than the reception of eternal life. The disciples already had eternal life, but their salvation was yet future. When Jesus said these words, before Pentecost, the Holy Spirit did not permanently indwell the apostles or other believers. After Pentecost, He did.
For unbelieving Palestinian Jews guilty of crucifying Jesus, they too would be justified by faith alone, but they would have to be baptized in order to receive the gift of the Holy Spirit (e.g. Acts 2:38; 22:16). Therefore Mark may be using the term salvation the way Luke and Paul use it, as a full package including eternal life, the forgiveness of sins, and the gift of the Holy Spirit.
Thomas Constable's Expository Notes on the Bible - Mark 16:16
16:16 This is a verse that some people believe teaches the necessity of water baptism for salvation. However Christian baptism elsewhere in the New Testament is always defined by an outward confession of belief in Jesus Christ. This verse also regards baptism as such. The second part of the verse clearly teaches that unbelief alone results in condemnation (cf. Mark 9:43-48), not belief and failure to undergo baptism.
In the first part of the verse one article governs both participles: "has believed and has been baptized" or "believes and is baptized" (NIV). This indicates the close relationship between believing and being baptized. However they are not inseparable (cf. Rom. 3:21-28: 1 Cor. 1:17; Eph. 2:8-9). Baptism is not a condition for salvation, but it is an important step of obedience for a believing disciple.