Drinking alcohol is haram (forbidden), and this goes uncontested in Islam (as far as I know). I was mulling over this question last night (just as a thought exercise):
Question: Which is worse: "believing drinking alcohol is not haram and not drinking alcohol" or "believing drinking alcohol is haram, but drinking alcohol"?
I can see an argument either way:
In the first case, we have a disbelief in a consensus opinion, which could be interpreted as kafir. At the same time, this is just a belief; it could be a simple, honest mistake. They might even be unaware of the consensus.
In the second case, the person is actively participating in something that is haram, and they know/believe to be haram.