The virginity of our Blessed Lady was defined under anathema in the third canon of the Lateran Council held in the time of Pope Martin I, A.D. 649. The Nicene-Constantinopolitan Creed, as recited in the Mass, expresses belief in Christ “incarnate by the Holy Ghost of the Virgin Mary”; the Apostles’ Creed professes that Jesus Christ “was conceived by the Holy Ghost, born of the Virgin Mary”; the older form of the same creed uses the expression: “born of the Holy Ghost and of the Virgin Mary”.
http://www.newadvent.org/cathen/15448a.htm
IMHO, if the creeds (aside from a capital letter here and there) are not believed whether one is Protestant or Catholic, then one is not a Christian.
Believing on the Lord Jesus Christ encompasses several other basics.
Of course one could just read the Bible and learn Mary was a virgin until Jesus was born. No councils needed.