The letters after a priest's name can indicate that he is both a priest and a religioius. Secular priests do not have initials after their names, because they are not consecrated religious. However, they can be an academic signature. For example:
DD = Doctor of Divinity
STD = Sacred Theology Doctor
JCD = Doctor of Canon Law
STL = Sacred Theology Licentiate
JCL = Licentiate of Canon Law
MARE = Master of Arts in Religious Education
M.Th. = Master of Theology
D.Min. = Doctor of Ministry
D.Th = Doctor of Theology
Many priests have academic degrees in theology, canon law, sacred scripture, religious education or ministry. It is not a requirement, but it is becoming more common.
Only priests with a doctorate degree can become a bishop. It cannot be an honorary doctorate. It has to be an academic doctorate. Pope Benedict has two, an STD and a PhD.
Pope John Paul had three: STD, PhD and another PhD
Br. JR, OSF
OSF = Order of St. Francis