Which countries will grant you citizenship automatically if you convert to a certain religion?
No state or nation that currently exists does that. However, the closest thing that exists to a law that allows you to gain citizenship based on your religion is the Law of Return in Israel, which states in Section 1: "every Jew has the right to come to this country as an oleh [immigrant]." It was passed in July of 1950 and gives Jews the right to come and live in Israel & gain Israeli citizenship. In 1970, the law also included people with one Jewish grandparent and a person who is married to a Jew, whether or not said person is considered a Jew by Orthodox Judaism standards. However, you don't instantly become a citizen. You receive a certificate stating you are a returning Jew and get three months to decide if you wish to be a citizen. Also, you can have your citizenship denied if you have "engaged in activity directed against the Jewish people, endangers public health or security of the state, or who has a criminal past that may endanger public welfare." Technically,simply being of Jewish ethnicity, even if you aren't religious, is another way to get the benefits of the Law of Return, but being part of the Jewish faith is also a relevant way to begin this process.