Art. 14 grants right to equality to all persons citizen or non-citizen against the arbitrary action on the part of the state. Art. 15 on the other hand prohibit the discrimination on grounds of religion, race, caste, sex or place of birth among citizens only.
It is notable that any classification on these grounds only will not be reasonable classification under Art. 14, therefore if a legislation is volatile of Art. 15, then it cannot be saved on the ground of reasonable classification which is the underlying principle of Article 14.