Yes, canonically, baptism makes one Christian. For the baptism to be valid it has to be done intending to indeed initiate into the Christian faith (not, for example, role playing as children might do), desired by the sponsors who voluntarily take the responsibility to inculcate the baptisee in the faith, be done in the name of the Father, the Son, and the Holy Ghost (or the Holy Spirit), and use water that touched the head of the baptizee. Anyone can baptize validly, even, in an emergency, a non-Christian, so long as the intention is there.
Apostasy is a complete rejection of faith in Jesus Christ. It is to be distinguished from heresy, that is falling to partial error of Christian doctrine.
Both subjects can have a dissertation written. Why do you ask?
Just wanted you to go.. on record..
which is honest.. many will not..