Christians always interpreted the Bible literally when it declares, "Baptism . . . now saves you, not as a removal of dirt from the body, but as an appeal to God for a clear conscience, through the resurrection of Jesus Christ" (1 Pet. 3:21; cf. Acts 2:38, 22:16, Rom. 6:3–4, Col. 2:11–12), and they have always interpreted that as WATER BAPTISM. Thus the early Church Fathers wrote in the Nicene Creed (A.D. 381), "We believe in one baptism for the forgiveness of sins."
And the Catechism of the Catholic Church states: "The Lord himself affirms that baptism is necessary for salvation [John 3:5]. . . . Baptism is necessary for salvation for those to whom the Gospel has been proclaimed and who have had the possibility of asking for this sacrament [Mark 16:16]" (CCC 1257).
The Christian belief that water baptism is necessary for salvation is so unshakable that even the Protestant Martin Luther affirmed the necessity of baptism. He wrote: "Baptism is no human plaything but is instituted by God himself. Moreover, it is solemnly and strictly commanded that we must be baptized or we shall not be saved. We are not to regard it as an indifferent matter, then, like putting on a new red coat. It is of the greatest importance that we regard baptism as excellent, glorious, and exalted" (Large Catechism 4:6).
Still, even though the necessity of water baptism is a normative, there are exceptions to water baptism. It is possible to be saved through "baptism of blood," martyrdom for Christ, or through "baptism of desire", that is, an explicit or even implicit desire for baptism. This applies to, say, the Good Thief on the cross. Thus the Catechism of the Catholic Church also states: "Those who die for the faith, those who are catechumens, and all those who, without knowing of the Church but acting under the inspiration of grace, seek God sincerely and strive to fulfill his will, are saved even if they have not been baptized" (CCC 1281; the salvation of unbaptized infants is also possible under this system; cf. CCC 1260–1, 1283).
I suggest people read the first seven links on this page:
http://www.catholic.com/library/sacraments.asp