Oznaim L'Torah on our parsha, 22:18. Why does Hashem approve of a gentile's korbon, even that of an idol worshiper? Rav Sorotzkin first restates an idea mentioned earlier, that this non-Jew is exhibiting faith in Hashem as the Supreme Ruler, despite his general worship of idols. If so, why not commend the sacrifice of an idolatrous Jew as well? The Oznaim L'Torah quotes the famous words of the Rama, Rav Moshe Isserles, in Shulchan Aruch Orach Chaim 156:1. "They (the gentiles) are not warned against shituf (pairing up idols with their worship of Hashem)". In other words, it seems from this Rama that the prohibition of idolatry for a non-Jew is if he EXCHANGES G-d for graven images. If he merely JOINS paganistic rites with his service of the Creator, he is not necessarily violating the commandments incumbent on him. This halacha is hotly debated (see Aruch Hashulchan who says it relates only to oaths of shituf, also see numerous sources brought in Shailos Uteshuvos V'shav Hakohen section 38), but this is what the Rama himself states, based on an earlier source. With this in mind, our matter becomes magnificently clear. A non-Jew, a ben Noach, is permitted to practice shituf according to this Rama. True, he is making a grave error in believing that idols have any power at all. But he still feels that Hashem is to be exalted and worshipped. Consequently, his korbon is accepted. On the other hand, it is well-known that a Jew is strictly forbidden to practice any form of idolatry, including shituf, a combined worship of Hashem and another god. Thus, the korbon of such a Jew is rejected.
Now you will understand better my question. It would seem A Xtian can still be keeper of the 7 mitzvot of the Noachide law and is not necessarily avodah zarah.
So am I reading this right?
Yes.