A - Thanks for the questions. We will look at what Baptism is first, because it is much more than many people think.
This is what Baptism does:
- Makes us children of God.
- Cleanses us from the guilt of original and personal sin.
- We die to our old lives and are born again.
- It is the doorway to entry into the Church.
Thus, we are freed from evil and made sharers in Christ. This means that evil had dominion over us before our baptism. Even if a child has not sinned personally, they share in the same fallen human nature and are in need of redemption. What they have inherited is condemnation, not salvation. Therefore, only by the mercy of God is anyone saved, not by our nature at conception.
As the Catechism says:
1237 Since Baptism signifies liberation from sin and from its instigator the devil, one or more exorcisms are pronounced over the candidate. The celebrant then anoints him with the oil of catechumens, or lays his hands on him, and he explicitly renounces Satan. Thus prepared, he is able to confess the faith of the Church, to which he will be "entrusted" by Baptism.The kind of exorcisms performed at baptisms are considered "minor" exorcisms. For a catechumen (adult preparing for baptism), this is a sign of their desire to renounce the devil and evil. For a child it is a sign of the parent's desire to renounce evil for the child. The Catechism tells us about exorcisms here:
1673 When the Church asks publicly and authoritatively in the name of Jesus Christ that a person or object be protected against the power of the Evil One and withdrawn from his dominion, it is called exorcism. Jesus performed exorcisms and from him the Church has received the power and office of exorcizing. In a simple form, exorcism is performed at the celebration of Baptism. The solemn exorcism, called "a major exorcism," can be performed only by a priest and with the permission of the bishop. The priest must proceed with prudence, strictly observing the rules established by the Church. Exorcism is directed at the expulsion of demons or to the liberation from demonic possession through the spiritual authority which Jesus entrusted to his Church. Illness, especially psychological illness, is a very different matter; treating this is the concern of medical science. Therefore, before an exorcism is performed, it is important to ascertain that one is dealing with the presence of the Evil One, and not an illness.Thus, at baptism, the minor exorcism is a rite performed in order to cast out the devil so that the person is made ready to receive Christ's life.
Now, on to the second question. The reason oil is used is because of what it symbolizes. Oil is used, when baptized, in two different kinds - the oil of catechumens and the oil of chrism. The oil of chrism symbolizes the reception of the gifts of the Holy Spirit and the oil of catechumens symbolizes the reception of strength and wisdom. It is part of the process of preparing a spiritual home for Jesus to reside in a person's life.
I hope this helps.