Is the consensus regarding the prohibition of shaving the beard established with certainty? And is it true that whoever denies the obligation of growing a beard and permits shaving it is not considered to have committed a major sin?

The question is incomplete and can be rephrased in full. As for growing the beard, many prophetic traditions (Hadith) have been reported, enjoining its growth and prohibiting its shaving, in opposition to the polytheists, Jews, Christians, and Magians. The Hanafis, Malikis, Hanbalis, and a view among the Shafi'is have held that shaving the beard is forbidden due to its contradiction of the prophetic command to grow it. Imam Ibn Hazm reported a consensus (Ijma') that shaving the beard is a mutilation (muthlah) and is not permissible.