This question revolves around the word ‘necessary’. Necessary for what?

Baptism is not necessary for salvation. It is only by grace that we are saved, not by any good works that we might do to try and earn God’s favour. This is quite clear in the Bible, in places like Ephesians 2:8-9 and Titus 3:4-7. In that sense then, baptism is not necessary.

However, Baptism is necessary for obedience. If you have come to Christ and want to be one of His disciples, then you have to do what it says in His Word. 1 John 2:3-6 says this quite clearly:

3 We know that we have come to know him if we keep his commands. 4 Whoever says, ‘I know him,’ but does not do what he commands is a liar, and the truth is not in that person. 5 But if anyone obeys his word, love for God is truly made complete in them. This is how we know we are in him: 6 whoever claims to live in him must live as Jesus did.

When the people at Pentecost ask Peter what they should do He replied:

“‘Repent and be baptised, every one of you, in the name of Jesus Christ for the forgiveness of your sins. And you will receive the gift of the Holy Spirit. 39 The promise is for you and your children and for all who are far off – for all whom the Lord our God will call.’” Acts 2:38-39. And when Jesus gave His final instructions to the disciples He commanded them to baptise those who would later become disciples: “18 Then Jesus came to them and said, ‘All authority in heaven and on earth has been given to me. 19 Therefore go and make disciples of all nations, baptising them in the name of the Father and of the Son and of the Holy Spirit, 20 and teaching them to obey everything I have commanded you.” Matthew 28:18-19

Baptism then is not necessary for salvation, but it is necessary for obedience to Christ. Therefore, if you have made a decision to follow Christ you must be baptised. If someone refuses to be baptised, they are refusing to obey Christ, and as John tells us in 1 John 2:3-6, we can see that they are not truly transformed inwardly.