Halachicly speaking how important is achdut amongst Jews and if it is important how are observant Jews supposed to behave in relation to the non-observant majority in Israel and around the world?

Achdut is a fundamental principal in Judaism. We pray for Achdut, most of our collective problems are because we are lacking Achdut. Yet, Achdut doesn't mean that we all think the same, look the same or agree about everything. Achdut means that even though we look, think, behave, believe, etc, differently, we still feel and behave as if we are one big family. We can say, and should say, clear and sound, that our non observant brethren are making a fatal mistake, we can say, and do say, that the only true and authentic Jewish way of life is the one of the written and oral Torah, but even if one fails to understand that, we love him and want to be a part of his life as well of him being part of ours, as Achdut is beyond that.