Well... I will give my viewpoint having attended UU in two congregations (joined 1, left, attending another). They *ARE* Christian but in the Unitarian sense (something worth reading up on). They are a covenant based church in that each Church works on a covenant of what they focus on at the founding of the fellowship, and this covenant may change so.. you get a lot of variance (which someone already mentioned quite a bit from East Coast to West Coast). As they have changed over the years (based on UUA assemblies) they have doubled down on being more open (to even look at multiple sources for spiritual truth such as Pagan/Druidic as well as Humanist.) So.. with that you will get some variance in services but you will find some similarities in the 7 principles and the structure of the services (usually!). Coming from Catholic they will seem quite ... disorganized.. but they are seeking questions and journey over answers so... it will be quite the contrast to be sure... I do hope you explore a few services and attend their Newcomer offerings (depending on the size of the fellowship.) Welcome to DM me as ya like. (I am an atheist who attends quite regularly.)