There are several definitions of the word “doctor,” but they all go back to the Latin word for “teacher.” But in the world of Catholic vocabulary, a church doctor is a confessor and a saint but not necessarily a martyr. The term confessor emanates from the Latin verb confiteri, “to declare openly,” and the title was given to those who led exemplary Christian lives but did not get the chance to die for their faith (martyrs).
The sanctity and learning of the doctors of the church means that they are an excellent examples of knowledge and inspiration towards two important dimensions of Christian life, faith and reason. In this case, the two dimensions are necessary, none of them is mutually exclusive in as much as faith deepens through knowledge, and knowledge get grounded in faith.
It’s either a pope or an ecumenical council (that is a council representing the whole church) can name a doctor, but in practice it has been popes who make the final declaration.
The church did not get enticed to proclaiming doctors until the 13th century when Pope Boniface VIII named the original four Doctors of the church which are: St. Ambrose (340-397 A.D.),St. Jerome (345-420), St. Augustine (354-430), Pope Gregory the Great (540-604). At the same time, Pius, a Dominican friar,(also named a brother Dominican) added Thomas Aquinas, to the list. Meanwhile, presently there are about 33 doctors of the Church.
It is a fact thing that every doctor in the church today has to be a canonized saint. Meanwhile, official sainthood is not so much a formal requirement, but it points to another defining characteristic of sanctity of doctorhood. It also points to one of their chief characteristics, what is usually referred to as an excellent learning.
Their expounding of the faith has been judged to be sound and of benefit to the whole church. But in everything, what matters is that you can lay your trust on a doctor when it comes to theological doctrine.