Apologist with a capital A refers to any of the Christian writers, primarily in the 2nd century, who attempted to provide a defense of Christianity against Greco-Roman culture. Greek Apologists include Justin Martyr and Clement of Alexandria. Latin Apologists of the 2nd century include Tertullian.
Many of their writings were addressed to Roman emperors and were submitted to government secretaries in order to defend Christian beliefs and practices.
Apologist with a small a refers to a person who tries to explain and defend something, especially a political system or religious ideas.
Also apologetics is the branch of theology concerned with the defense or proof of Christianity.
The person who makes a will (or 'testament') is a testator.
A composer is a person who writes music.
A person who writes laws is called a legislator or a lawmaker.
The person who writes a check is called the payor. Or in the case of my brother, a sucker, when he writes another check for his girlfriend.
A Philosopher
An autobiographer is a person who writes the story of his or her own life.
A person who studies or writes history is called a historian.
A person who writes the libretto for an opera is simply referred to as the librettist.
testator
A person who writes plays is known as a playwright. An example of a playwright is William Shakespeare.
biography
An author.