Outside the Catholic Church, there are believers in Jesus, who hold that baptism is only a public manifestation of one’s inner faith, a symbolic identification with Christ’s death, burial and resurrection. Accordingly, children are not baptized. Each individual, when becoming an adult and believing that he has accepted Christ in his life, make a public announcement of his newly found faith. This public announcement is referred to as a baptism.
Contrary to this, the Catholic Church holds that the Sacrament of Baptism is absolutely necessary for salvation after the proclaiming of the Gospel to everyone, both children and adults.
Throughout the ages, various Popes have proclaimed the necessity of baptism for salvation.
Pope Zosimus (417 - 418) approved a teaching of a local council at Carthage which condemned those who denied baptism to newborn infants.
Pope Celestine I (422 - 432) taught that both children and infants need the sacrament of regeneration.
Pope Innocent II (1130 - 1143) and the Council of Lateran IV condemned those who denied the baptism of children.
Pope Innocent III (1198 - 1216) condemned those who said that baptism of children was useless.
Pope Clement V (1305 - 1314) in the Council of Vienne defended the necessity of baptism of children.
Pope Eugene IV (1431 - 1447) in the Council of Florence defended the necessity of baptism of children.
The Council of Trent (1545 - 1563) affirmed the necessity of the Sacrament of Baptism for salvation.
Over and above the aforementioned Catholic Church proclamation, we find references in the Holy Bible that support the necessity of baptism for salvation.
Jesus said, “Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit...” [Mt 28:19]
“Jesus answered, ‘Very truly, I tell you, no one can enter the Kingdom of God without being born of water and Spirit.” [Jn 3:5]
“He saved us, not because of any works of righteousness that we had done, but according to his mercy, through the water of rebirth and renewal by the Holy Spirit.” [Tit 3:5]
When Jesus commanded the Apostles to go into all the world and to preach the Gospel to the whole creation, that those who believe be baptized and saved, Jesus made faith an important part of the Sacrament of Baptism (C.C.C. # 1226) along with the forgiveness of sin. [Mk. 16:15-6] (C.C.C. # 985)
"And Jesus said to the Apostles, 'Go into all the world and proclaim the good news to the whole creation. The one who believes and is baptized will be saved; but the one who does not believe will be condemned.'" [Mk. 16:15-5] (C.C.C. # 977)