Allama ibn Hazm in his short treatise “an-Nubaz al-kafiya fi akhamil usul ad-deen” said:
“Contrary to this, the one who goes against agreement of companions, and deliberately transgress it, if there is a proof (for him established) and he was informed and (he) still persist – he would be consider as disbeliever”.
Source: “an-Nubaz al-kafiya fi akhamil usul ad-deen” p 21
And he also said:
Each person which comes after and he act in accordance to agreement of companions and doesn’t contradict to it, consider believer. The one who goes against agreement because lack of knowledge regarding such – his words are not taken into account. The one who acts contrary to it, while he knows, and persist in this – is disbeliever.
Source: “an-Nubaz al-kafiya fi akhamil usul ad-deen” p25