Some Christians believe in and live according to the Noachide Laws. According to Judaism, doesn't this qualify them for heaven? I know blasphemy is in that list, but Christians Genuinely believe that Jesus is the messiah.
Blasphemy is not the problem — it refers to cursing G-d with His name (Rambam, Melachim 9:3). The problem is idolatry, and many hold that Christianity is idolatry. However, the Rambam writes elsewhere (ֲMelachim 8:11) that a non-Jew only has a portion in the World to Come for doing the things he is commanded in if he does them because G-d commanded them; if he does them because they seem to be good things to do, even if G-d didn't command them, then this doesn't apply.
There any many passionate feelings and positions regarding this issue. There is no one correct answer. Some say Christianity is shituf (acceptable idolatry?) yet others feel it is simply idolatry. I even heard an interpretation saying that G-d dislikes the notion of a partnership (Jesus) more than idolatry.
Be it as it may, I think that Maimonides criticized Christianity. I think what bothered him the most was the notion that there are three gods. He felt strongly that this is philosophically impossible. Nevertheless, Moslems who worship Allah (one G-d) are considered Noahide. Can Chrisitan non-Jews practice the Seven Noahide Laws?
Surprising as it may sound, the Bible actually never make mention of these commands, known as the Seven Noahide Laws, commandments which G-d gave Noah after the flood. The seven Noahide commandments are part of the “Oral Torah,” the unwritten parts of the Torah. Although there is no source for this, these laws are explicable in nature and could be considered the basic laws of humanity, applicable to all human beings. But people should not observe these laws simply to have a place in “the world to come.” I think that G-d wants people to act as if there is no G-d. That is to say, if people recognize a need to be done, people should help people and society, and act as if there is no G-d and nothing will be done to resolve the problem unless they do what needs to be done. For the sages taught in Avot: do not observe the mitzvot shelo lishmah: “not for the sake [of the Torah] itself.”
Additionally, people can't observe the commands because G-d said so since G-d is incorporeal. Maimonides explains in his Guide of the Perplexed 1:65 that whenever the Torah says that G-d “said” or “spoke” something, it should be read as “thought” or “willed,” for G-d has no mouth with which to speak. God has no vocal cords. For example, in Genesis 1, G-d did not speak or utter words during the act of creation, but “willed” creation into existence. Similarly, the conversation G-d had with Noah was a dream. A person does good acts because it is the right thing to do, as it is the will of G-d, and avoids doing evil because it is wrong.
In any event, even those who stress the importance of the commands, the mitzvot, all agree that the ultimate goal of humans is to develop their intelligence. Maimonides felt that knowing about G-d's creations (the laws of nature) and an understanding of how they function (the impact of natural laws) is essential. For Maimonides insisted that the basic human duty is to know about G-d; and since it is impossible to know the essence of G-d, he stressed the need to understand the laws of nature that G-d created or formed so as to improve themselves and society (Guide of the Perplexed 1:1).
Thus, any observance of commands is a means to help people better understand G-d. If a non-Jew observes the Noahide commands and knows their divine origin, but fails to acquire knowledge, they have failed and in this respect, is no different from a Jew who does not develop their mind.