It's one of the most controversial doctrines in all of Christian history. Why?
In the first place, baptism is a symbolic representation of a biblical truth, and symbols are inherently slippery. A symbol just naturally lends itself to individual interpretation (the favorite altar at which mankind loves to worship), thereby lulling us into fanciful and obscure meanings that bear little resemblance to the intended reality. With today's prevailing attitude being "every man doing what seems right in his own eyes", it is naturally assumed that baptism pictures no absolute truth for which we are accountable.
Secondly, because baptism is something done, an action taken, it easily morphs into a work that is thought to contribute to one's salvation. Baptism is one of many tools that Satan uses to re-enforce the natural (and fatal) assumption that one can gain salvation and access to heaven by good deeds, transforming salvation into that which is earned (and thus deserved).
An extension of the second point relates to terminology. Multiplied millions consider baptism a sacrament. A sacrament, by its very name, is thought to impart grace. And, as it is true that grace is empowering, it follows that a sacrament would complete or contribute to the salvation of an individual. From this sacramental perspective, the cry of the Savior from the cross on Calvary would be a lie when He said: "It is finished!"
The truth is that baptism is an ordinance. As the 29th chapter of The Baptist Confession of Faith of 1689 reads: "Baptism is an ordinance of the New Testament instituted by Jesus Christ. It is intended to be, to the person baptized, a sign of his fellowship with Christ in His death and resurrection, and of his being engrafted into Christ, and (on the basis) of the remission of sins. It also indicates that the baptized person has given himself up to God, through Jesus Christ, so that he may live and conduct himself 'in newness of life.'
Hence, modeled by the Lord Jesus Himself (Matthew 3:16), commanded by Him to all His disciples (Matthew 28:19), and expounded by the apostle Paul (Romans 6:3-5), baptism is to be an expression of joyful obedience to a loving Lord, following His example by immersion, and marked as the public declaration of a holy, new life lived in the power of the Holy Spirit.