The question of whether baptism is necessary for salvation is a highly denominational one, meaning that different interpretations exist among different congregations. However, water baptism, in itself, is not generally regarded as absolutely necessary, nor is it always considered the only acceptable form of baptism.
Drawing on Gospel verses such as John 3:5, which states that, "the Lord himself affirms that baptism is necessary for salvation," the Catholic Church holds that baptism is essential for the salvation of the soul. However, it shares in the more widely held Christian belief that water baptism is normative, or traditional, rather than absolute. Other acceptable forms of baptism include those of the martyr or even the internal desire to be baptized.
Among Protestant denominations, views get more complex. Nonetheless, Protestants generally place greater salvific emphasis on faith, concluding that it alone is more important than any other element, ritual or tenet on the road to salvation. For many Protestants, baptism itself is an initial step into the covenant of grace, as described by Matt Slick at Christian Apologetics and Research Ministry.
While these different attitudes towards baptism and salvation have been affected by some more modern theological innovations, the basic division between Catholic and Protestant attitudes toward the question stretches back to arguments that animated the 16th-century Reformation. That is, specifically, the Protestant belief that any criteria for salvation outside faith, such as rituals like baptism, imply a salvation attained through works. Said Protestant views explicitly identify the doctrine of works with the Catholic Church and its institutional character, something early Protestants fervently sought to reject.