The Supreme Court in a ruling that struck down a law that required a notary public to declare a belief in God noted:
Among religions in this country which do not teach what would generally be considered a belief in the existence of God are Buddhism, Taoism, Ethical Culture, Secular Humanism and others.The breadth of the term "religion" was already dealt with by lower court rulings for decades at that point, addressing issues of conscientious objection and tax issues, including in cases involving ethical culture. An aside. The "belief" is also not necessary to religion (if so, "religious belief" might be redundant), suggesting a state of mind and emotional attachment that might not be present in some religions, including those focused on action. Anyways, does this mean that the state violates the Establishment Clause by promoting the principles of "secular humanism" as some suggest? No, unless the state promotes the idea that one should not believe in God. As with promoting kindness the others, it's okay to further things some religions support.
The Supreme Court later tried to set a limit here by saying that "choice was philosophical and personal, rather than religious, and such belief does not rise to the demands of the Religion Clauses." Thoreau was cited. Justice Douglas, accepting Seeger as stating a constitutional rule, argued that this was too limited -- "based upon a power or being, or upon a faith to which all else is subordinate or upon which all else is ultimately dependent" would overlap. A look at various lower court opinions, as I have noted in the past, suggests there is not really too much space between these things, especially if the former is applied liberally.
Some worry about such an open-ended definition of religion. The fact remains that it a complete account of what society recognizes as "religion" is not limited to belief in God and the afterlife,* even if that was how things were originally understood when the First Amendment was ratified (if not something even narrower). People have all types of religious beliefs and practices, the term at times used to explain a way of life as much as anything else. Thus, protection of freedom of religion, or more completely, freedom of conscience, should be understood and respected broadly. The breadth also suggests that for purposes of the Establishment Clause, the test is not quite as broad, at least, if the state is not seen as endorsing or furthering a particular religion over another. Or, inhibiting (such as favoring one view of abortion over another) the free exercise thereof.
There is some "play in the joints" there. Happy Hanukkah.
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* The mere belief that there is a God or some such being/force is not much of a religion, particularly if someone doesn't put any meaning to such a being's existence. The same might be said about lack of such a belief. "Atheism" is not a religion, unless it is expressed in such a way that it amounts to such. Some atheists are in effect evangelists and not accepting the existence of God is just part of an overall belief system though I realize the terminology might be deemed insulting to some.