Catholics believe in seven sacraments, some of which cannot be undone once they are done: Baptism, Confirmation, Eucharist, Reconciliation, Marriage, Holy Orders, and Anointing of the Sick. Baptism, Confirmation, Marriage and Holy Orders are one time and permanent.
Once Baptized, a person is permanently marked by God and will never lose that mark, even if they repudiate everything Christian and live a debauched life, never repent and end up in Hell by their own choice–the mark will remain. The same is true of Confirmation, Marriage and Holy Orders.
Marriage and Holy Orders are entries into a state of life. This is a condition which is permanent and irrevocable. Once a person enters into either of these Sacraments their vocation is set and they cannot go back. This is why I write so often on vocations and just as frequently mention marriage.
Confirmation completes Baptism and gives them the recipient the grace to serve God into adulthood. Provided of course they choose to live that way.
Three Sacraments are called the Sacraments of Initiation, these are Baptism, Confirmation and Eucharist. Adults entering the Catholic Church from an unchurched, unbaptized background receive all three at the same time. In the Eastern and Orthodox Churches, infants are given all three sacraments of initiation shortly after birth. In the West, due to problems with few bishops and difficulty traveling over vast diocese, the three sacraments of initiation became separated in practice so that infants receive Baptism, teenagers receive Confirmation; in between, out of their traditional order, Eucharist is given to children–unlike adults entering as adults. Baptized adults entering full communion are given Confirmation and then Eucharist, so like the unbaptized entering the Church the Traditional order of the Sacraments of Initiation is maintained.
Sacraments that repeat are: Reconciliation, Anointing of the Sick and Eucharist.
Reconciliation is for whenever needed. Children go to their first Confession (another name for the Sacrament of Reconciliation) sometime around the age of reason (7 years). This is a method of fulfilling the commandment to “confess your sins to one another” but without the risk that gossips will spread your every sin around.
Anointing of the Sick (also called last rites) can be given for illness as well as right before death.
Eucharist is part of the Mass. This is the living presence of Christ Jesus, Body, Blood, Soul and Divinity given to us to help us become like Jesus, and to unify us into the Body of Christ. Only a Catholic in a state of grace is to receive the Eucharist. It is a curse to receive unworthily and a non-believer in the sacramental reality will not receive worthily due to their lack of proper belief. Because of this it is not right to allow someone to bring spiritual harm to themselves by receiving without proper belief.
All seven Sacraments are essential for Catholics to understand and take advantage of the privilege to receive them. These means of grace in our lives are part of our vocations.