I heard that if a Jew doesnt keep Shabb-t he is concidered as if a non-Jew and in some things even worse. in Halaha there are examples - if a non Shabb-t observing Jew wants to bring a sacrifice in the temple he is rejected while a non-Jew who keeps the 7 laws isnt (maybe not sure could be even an idolater, dont remember exactly).
you can take interest from that kind of Jew, but we dont becuase lets say - you give a loan for 3 years with 5 % interest, then if he in the middle or whateva does tishuva then you cant take interest from him, etc.
also about the brit, I heard that becuase of some sexual sins like sleeping with a goya (non-Jewish women) when a jew who did this (and didnt repent) goes to the next world, he is rejected and he gets a forskin like a non-jew, and Avraham Avinu doesnt let him in heaven. (he is reincarnated, or worse, etc.)