The Sacrament of Baptism illustrates what has been said in "The Sacraments in General." The outward sign of this Sacrament which is readily perceived by the senses, is the pouring of water and the saying of the words: "I Baptize you in the name of the Father, and of the Son, and of the Holy Spirit."

The words: "I baptize," and the pouring of the water, signify cleansing by washing; and so indicate the spiritual effect baptism has on the soul, washing and cleansing it from sin, by the pouring into the soul, by Almighty God, of sanctifying grace. Jesus Christ Himself, speaking to the Samaritan woman, called sanctifying grace "a fountain of water, springing up into life everlasting" (John 4:14).

Baptism is the first of the Sacraments to be received; because before receiving any other of the Sacraments, we must be members of the Church; and it is by receiving Baptism that we become members of the Church. Baptism was referred to by Jesus Christ when He said to Nicodemus; "Unless a man is born again of water and the Holy Spirit he cannot enter the Kingdom of God." (John 3:5.) Christ instituted the Sacrament of Baptism when He said to His Apostles: "Go therefore, teach all nations, Baptizing them in the name of the Father, and of the Son, and of the Holy Spirit." (Matt. 28:19.)  The Church teaches that Baptism is necessary for all; for infants as well as adults.

The writings of the early Fathers of the Church assure us that Baptism was administered to all, adults and infants. Thus Origen writes: "The Church received this tradition from the Apostles to give Baptism even to infants. (In Ep.ad Rom.) A few German fanatics in the sixteenth century, calling themselves Anabaptists, raised their feeble voices against the voice of all Christendom, and refused Baptism to infants. From them the modern Baptists receive their doctrines.

"As regards children who have died without Baptism, the Church can only entrust them to the mercy of God, as she does in her funeral rites for them. Indeed, the great mercy of God who desires that all men should be saved, and Jesus' tenderness toward children which caused him to say: "Let the children come to me, do not hinder them." allow us to hope that there is a way of salvation for children who have died without Baptism. All the more urgent is the Church's call not to prevent little children coming to Christ through the gift of holy Baptism."  1261, Catechism of the Catholic Church.

A study of the history of the Church shows us that there have been three methods of administering baptism adopted in different places at different times: Baptism of immersion or dipping; of aspersion or sprinkling; and of pouring the water. All are valid methods of administering the Sacrament. But for special reasons, pouring of the water is the method prescribed throughout the Latin Church today.

Everyone should know how to baptize, as the time may come when Baptism administered in an urgent case, may save an infants soul.  The priest is the ordinary minister of Baptism; but in case of necessity any one can administer Baptism, and without the usual ceremonies.  As baptism is so necessary, parents should see that their children are Baptized as soon as possible after birth.

The question may arise:  If Baptism is necessary for salvation, how can a person - not baptized, who has never heard of Baptism be saved ?

The answer is that there are three kinds of Baptism: (1) Baptism of Water. (2) Baptism of Blood, which takes place when one who is not yet baptized suffers martyrdom for the Faith; as happened to many of the early Christians. (3) Baptism of Desire, which has effect when a person loves God, and so would do His will if it were known. This implies that if such a person knew of the necessity of Baptism, he would receive it; and so he has an implicit desire of Baptism. This is sufficient for his salvation.  Baptism can be administered only once. It leaves an indelible mark on the soul which sets apart the one who receives it as a Christian forever. This will be for his greater glory in Heaven if he is saved; or for his greater shame and torment in Hell if he is lost. But Baptism of Water alone is the Sacrament. Baptism of Blood and Baptism of Desire do not make one a member of the Church, nor do they mark the soul with an indelible character.

Source: Rev. W. Frean. Commentary on the Catechism.

The effect of Baptism is to remit Original Sin (for adults it remits both Original and actual sin as well as all guilt and punishment which they incur, which means that if an adult should die after receiving Baptism, and has not sinned since receiving this Sacrament, goes strait to Heaven, No Purgatory!)

No kind of punishment must be enjoined for past sins upon those just newly baptized. Therefore the effect of Baptism is the opening of the gates of Paradise.

Source: St. Thomas Aquinas, God's Greatest Gifts.