What are beliefs? They are a pre-determined rule book that we all have our own copy of; each is different and unique to us. We form our beliefs when we are very young, they come from influencers who usually look after us and from their behaviours and things we learn along the way and from that we decide "which rules" we live our life by.
When we step into judgement of another person, we are comparing their behaviour to what we deem or believe is right or wrong. Our beliefs, or rule book, determines how we react. When we find ourselves in conflict, it is usually because we have judged someone.
Judgement stems from our beliefs.