The definition makes no declaration about the Church's belief that the Blessed Virgin was sinless in the sense of freedom from actual or personal sin. However, the Church holds that Mary was also sinless personally, "free from all sin, original or personal". The Council of Trent decreed: "If anyone shall say that a man once justified can sin no more, nor lose grace, and that therefore he who falls and sins was never truly justified; or, on the contrary, that throughout his whole life he can avoid all sins even venial sins, except by a special privilege of God, as the Church holds in regard to the Blessed Virgin: let him be anathema."
What was the name of the ecumenical council that made the decision ?
The Council of Trent