I gave all the quotes showing you how denying BOD is a mortal sin.
Catechism of St Pius X :
29 Q: What is required for a sin to be mortal?
A: For a sin to be mortal three things are required: (1) Grave matter, (2) Full advertence, (3) Perfect consent of the will.
30 Q: When is the matter to be considered grave?
A: The matter is grave when the thing under examination is seriously contrary to the laws of God and His Church.
31 Q: When is there full advertence in sinning?
A: Full advertence in sinning is had when we know perfectly well that we are doing a serious evil
32 Q: When is perfect consent of the will verified in sinning?
A: Perfect consent of the will is verified in sinning when we deliberately determine to do a thing although we know that thing to be sinful.