Lemon Test: Secular Purpose Prong

Here is how one commentator describes the secular purpose requirement under the Lemon test:

"The secular purpose element of the Lemon test requires that a law have a nonreligious purpose. This does not mean that the sole purpose of a challenged law must be secular, but only that the government must articulate a nonfrivolous secular purpose for the law. Thus, a law that is passed, in part, to advance religious doctrine is not necessarily invalid under this element if there is also a secular purpose for the law. In general, the Court tends to be deferential toward the government in applying this element. However, if the facts establish that the sole or predominant purpose of a law is religious, the Court will strike down the law on this basis."

Is the "purpose" prong of Lemon a structural limitation on government or is it a "liberty-advancing" test?