Here's what I think I have learned from studying and reflecting on my faith tradition while learning to appreciate others:
Faith traditions can begin for all sorts of silly reasons—for example, attributing weather to the moods of one or more temperamental, very powerful personal beings with whom we can bargain. If that's how they remained many of us would ignore or even ridicule them, as we learn more about how the world works.
But as detractors often fail to note, that's not how they remain. As faith traditions spread and endure they always attract more reflective followers who develop a more contemplative faith that is not silly at all.
Contemplative faith happens whenever something bounded seems open to the boundless, as both boundlessly here and boundlessly good: what may first seem most remotely abstract or forbidding comes to seem most intimately concrete and inviting.
Faith traditions often begin by responding to their surroundings in personal terms, but as they become more contemplative they all turn to more-than-personal terms (like "the boundless" or "being" or "being-here-now"); some remain theistic (that is, they retain words like “God”); some become nontheistic, avoiding terms like “God” altogether; hardly any seem to turn to less-than-personal terms, however.
All come to recognize that, while no terms adequately describe the boundless, some terms are more informative than others (e.g., "the boundless" or "being" or "being-here-now" seems more informative than "fingernail clippings").
Faith traditions that remain theistic come to realize that terms for personal and interpersonal presence (being-here-now) are themselves elusive terms that defy objectification, that is, there seems to be something boundless about them, making them compatible with the more-than-personal.
To paraphrase St. Paul, the boundless may be one from which, through which and in which all things are (Romans 11:36), but so, in a way, are you. All you'll ever know of what isn't you happens in and through you, and in a way even from you (though largely unexpectedly). You can never be an object for yourself the way you can be for others, not if you're you. Nor can I. Nor can we. There's something boundless about being you, me and us, here and now. Physics and other natural sciences, as we know or can imagine them, have nothing to say against this (and of course vice versa).
Such insights may never occur to casual theists, that is, most theists (!), but they have occurred frequently to contemplative theists for centuries, and if they had not occurred frequently it is doubtful that the three major theistic traditions would even exist today in any recognizable form.
Theistic arguments or "proofs" generally try to show how anything bounded opens to the boundless as its all-inclusive setting. Anselm: our very thought of the boundless, if it's really about the boundless, opens to the boundless beyond all thought. Aquinas: bounded causation opens to boundless causation; bounded goods open to boundless good.
Such attempted proofs are persuasive when something bounded already seems open to the boundless, but hardly ever otherwise, and they provide no support for casual theists, only for contemplative theists.
In each of the three major theistic traditions there is a different focus on something bounded that seems to open most effectively to the boundless: in Judaism, a holy people; in Christianity, a holy person-in-communion, in Islam, a holy recitation.
Nontheistic traditions likewise focus on something bounded, often a cluster of practices, that seems to open most effectively to the boundless. (For them the boundless is more like here-and-now than I-and-you. But does being-here-now ever happen without I-and-you? Has it? Can either nontheists or theists afford to be competitive or exclusive here?)
All faith traditions can turn dangerous when they forget a) that opening to the boundless is itself a bounded opening to the boundless, and b) that the point of focusing contemplatively on something bounded is to open us not to itself but to the boundless. When they forget this they abuse not only others but themselves.
This is why people in and beyond specific faith traditions need to enact protections against such abuses (keeping religions disestablished, for example). Such protections are in everybody's interest.
When devotees of differing faith traditions remember all this, however, they begin to open themselves to differing, bounded ways of opening to the boundless. They begin to fulfill their deepest insights. They deserve recognition and encouragement for this, not indifference or disparagement.
I say all this, mind you, as someone who opens to the boundless in terms of "the Communion of God's Spirit in Jesus Christ." I've been doing that for over forty years. For me those terms, while bounded, seem to open me most effectively to the boundless, partly because they intentionally convey a certain openness and instability (not God alone, not Spirit alone, not Jesus or the Christ alone, not the sum of all three, but their Communion—one that includes you and me and others here and now). That Communion is intrinsically open to differing, bounded ways of opening to the boundless, since communion without such openness is no communion at all.