Level 1 (topmost) is scientific knowledge. Revealed and studied by direct observation of nature and life.
Level 2, below Level 1, is the written Torah (TaNaKh). Most of it is accessible to study by anyone, Jew or gentile and siginificant lessons in what G-d wants and how to relate to others can be derived from it. One major proviso is that it msut be studied with the classical commentaries, such as Rashi, et al. to get first of all, the proper meaning and secondly, to understand it and be able to apply it. It will also help, given enough depth, in scientific studies.
Level 3 is the Oral Torah. Without the Oral Tradition, the study of the Tanakh is limited, the Oral Tradition adds color and dimension, at the very least, and the study of the Talmud, which is part of the Oral Tradition, by most opinions, is a significant component of the commandment to study Torah. This level mostly pertains to Jews as the gentiles’ mission is “yishuv ha’olam”, i.e. settling or making the world civilized through the fulfillment of the Seven Noahide Laws and for that, the preceding two levels are more than adequate.
Level 4 is the hidden Torah, the Kabbalah. Accordingly, its study supplements the Oral Torah. Hence, just as the study of the Oral Torah devolves on Jews only (for the most part), likewise, the study of Kabbalah is also restricted.
Level 1 is accessible by default to everyone. Levels 2 and 3 are conditional, i.e. open to any Jew but limited to gentiles insofar as their need to fulfill their Noahide obligations, that is, study of only those parts of the Oral Torah that help with the Noahide cammandments. Otherwise it’s considered infringement on someone else’s heritage. Level 4 is concealed in general and one needs to expend a great deal of effort to attain it and properly use.
Overall, the study of each succeeding level without mastery of the previous makes (with the exception of Level 1, the sciences) no practical sense and is of limited value. Realistically speaking, the study of pure Kabbalah is not easily accessible or practical even to most Jews as there’s plenty to achieve in levels 2 and 3 and to most people Kabbalah is esoteric and is poorly understood. The one exception is the study of chassidus which can be viewed as a subset or superset (depending on one’s view) of Kabbolah which, unlike pure Kabbolah itself, is quite necessary for the practical performance of mitzvoth and maintenance of a relationship with G-d.
Kabbalah demands of its students fairly stringent requirements. And even those who are able to read and somewhat comprehend the works of Kabbalah, if done without the proper approach, cannot be considered real kabbalists.