A Master of Divinity (MDiv) is a “professional” degree offered by a seminary that prepares one for a career in ministry or admission into a seminary DMin program. The DMin is awarded after completing the required course work and writing of a ministry-oriented dissertation. Seminary programs are usually accredited by the Association of Theological Schools (ATS).
A Masters of Arts (MA) is an “academic” graduate degree offered by a college or university that prepares one for a career in a related field or admission into an American university PhD program. Such degrees are approved by regional accrediting agencies.
Students with a seminary MDiv in hand who wish to enter a university PhD program will likely be asked to earn an “academic” MA from an accredited college/university prior to being considered for admission.