"Peter said to them,
Repent and be baptized, every one of you,
in the name of Jesus Christ for the forgiveness of your sins;
and you will receive the gift of the Holy Spirit"  ~Acts 2:38

The Sacrament of Baptism is often called "The door of the Church," because
it is the first of the seven Sacraments, not only in time, but also in priority.
The reception of all other Sacraments depend on it.

Baptism is the first of the three Sacraments of Initiation,
the other two being the Sacrament of Holy Communion and the Sacrament of Confirmation.  

Once Baptized, a person becomes a member of the Church.