Well, it depends on how you define Reformed. Historically, "Reformed" described a whole system of beliefs, from paedobaptism to Calvinism. If you were to talk to some folks in historically Reformed churches they will use phrases such as "Big "R" Reformed" or "Truly Reformed" or "TR" for short. In the 1960s the Reformed Baptists gained traction from a movement in Carlisle, PA. While the term has caught on, Calvinistic Baptists are not really "Big R" Reformed. They reject paedobaptism as well as the Westminster Standards. They also reject Presbyterian church polity, which is another aspect of "Big R" Reformed. Many Reformed Baptist do hold to Covenant Theology, but their differences with Presbyterian "Big R" Reformed churches are pronounced. The only thing "Reformed" about John MacArthur is his soteriology. If you ask many Presbyterians and Dutch Reformed whether any Baptist can be Reformed, you will get a big fat "Pfft."