Man: I'm a Gentile. I don't need to keep Shabbat, right? I mean, you don't really care, do you?
G-d: "...keep my sabbaths, and choose the things that please me and take hold of my covenant," [Isaiah 56:4]
Man: Well maybe it pleases you...but what's in it for a Gentile like me?
G-d: "...the sons of the strangers...that keep the Sabbath..them will I bring to my holy mountain and make them joyful in my House of Prayer..." [Isaiah 56:6-7]
Man: For your information, I have it on good authority from First Fruits of Zion, makers of very religious and very pretty looking books, that I'm not required to keep Shabbat. So I'm quite sure that all Gentiles, whether sabbath-keepers or sabbath-desecraters, can go to your House and be joyful.
G-d: "...all who keep the Sabbath without desecrating it...these I will bring to my holy mountain and make them joyful in my House of Prayer."