Why can't I be re-baptized as a Catholic?
I was baptized in a Lutheran church as a baby and would now like to enter the Catholic Church. Because I have not been living a Christian life until recently, I very much want to be re-baptized but have been told that the Catholic Church does not re-baptize. Why?
Baptism is a once-for-all sacrament that washes away original sin, gives sanctifying grace, and imparts a supernatural character upon the soul that makes a person a Christian. An attempt to "redo" a valid baptism would be useless: The second baptism would not "take" because the first was valid. Furthermore, it would be an objective sacrilege because it would cast aspersion on the validity of the first baptism.
Even if you have not lived a Christian life until recently, if you were validly baptized then you are a Christian. Your status as a Christian does not depend upon how well you have lived. The proper sacrament to wash away personal sins you have committed since your baptism is confession.