The purpose of the Establishment Clause is not to safeguard individual religious rights. That is the role of the Free Exercise Clause, indeed its singular role. The purpose of the Establishment Clause, rather, is as a structural restraint on governmental power. Because of its structural character, the task of the Establishment Clause is to limit government from legislating or otherwise acting on any matter "respecting an establishment of religion." The powers that fall within the scope of the foregoing clause (denied to government, hence within the sole province of religion) and the powers outside this clause (hence, authority vested in civil government) await elaboration below.
Carl H. Esbeck, Differentiating the free exercise and establishment clauses, 42 J. Church & St. 311 (2000)