Israel only allows Jews to become Israeli citizens.
In 1999, the Supreme Court of Israel ruled that Jews or the descendants of Jews that actively practice a religion OTHER than Judaism would not be allowed to immigrate to Israel as they would no longer be considered Jews under the provisions of the Law of Return.
Israeli legislators chose to make a clear distinction between the Law of Return, which allows for Jews and their descendants to immigrate to Israel, and between Israel's nationality law, which formally grants Israeli citizenship based on the Oleh's certificate. In other words, the Law of Return in and of itself does not determine Israeli citizenship; it merely allows for Jews and their eligible descendants to permanently relocate within the territory of Israel.
The state of Israel does, however, grant citizenship to any applicant who immigrated to Israel via the Law of Return if the applicant so desires, though this is not mandated by the Law of Return itself.