"if someone acts contrary to the law of God because of an erroneous [but of course binding] conscience he is not excused from sin. . . ."
"an erroneous conscience [though sufficient to bind] is insufficient to absolve when it sins from its own error, as when it errs about things it is held to know [(circa ea quae scire tenetur)]."
Thomas Aquinas, De veritate q. 17 a. 4 arg. 8; q. 17 a. 3 ad 4 (Thomas Aquinas: selected writings, ed. & trans. Ralph McInerny (New York, NY: Penguin Books, 1998), 231), italics mine. "That conscience binds means that when one does not follow it he incurs sin, not that one following it does the right thing" (q. 17 a. 4 Resp.) or is free to persist in error.