"Though I've never understood how God could expect his creatures to pick the one true religion by faith—it strikes me as a sloppy way to run a universe." (13.46)
Jubal doesn't necessarily disregard faith, but his inner rationalist finds the notion less than convincing or, at the very least, not fulfilling.
But with bleak honesty Jubal admitted that the Fosterites might own the Truth, the exact Truth, nothing but the Truth. The Universe was a silly place at best… (14.109)
Not being able to know the truth means there is always the possibility that claims of truth that Jubal finds unconvincing might still be right. It's Jubal's "best to keep an open mind" philosophy at work.
"Thou Art God," Mike repeated serenely. "That which groks. Anne is God. I am God. The happy grasses are God. Jill groks in beauty always. Jill is God. All shaping and making and creating together—" (14.180)
Mike's Martian heritage translates this as an answer for who or what is God. Very similar to pantheism, the idea is that God and the universe are identical, one and the same.
"Everything always is—and the more it changes, the more it is the same. Now slot machines—Ever see a Bingo game in church?" (24.29)
There's nothing new under the sun, but those non-new things can still be tweaked. Jubal sees a line of similarity running through and connecting all different religions.
"All those religions—they contradict each other on every other point but each one is filled with ways to help people be brave enough to laugh even though they know they are dying. […] Jill? Is it possible that I was searching them the wrong way? Could it be that every one of all religions is true?" (29.182)
Mike isn't quite on board with Jubal. Whereas Jubal believes any religion might own the truth, Mike sees all religions as having a piece of the truth. Then the question becomes which religions and what pieces.
"We must give the Devil his due. Mike does believe and he's teaching the truth as he sees it." (33.108)
For Jubal, it doesn't really matter if a religion is teaching a truth he can or can't verify. The question is whether or not they honestly believe it is the truth. They might be mistaken, but at least they're not a bunch of con men.
"I grok this: we don't actually need Mike. You could have been the Man from Mars. Or me. Mike is like the first man to discover fire. Fire was there all along—after he showed them how, anybody could use it… anybody with sense enough not to get burned with it." (35.104)
Ben shares his realization with Jubal. If something is a truth, even a religious truth, then it can be open to everyone. According to this line of thinking, prophets, pastors, priests, and messiahs may help you grasp the truth or be there for support, but they're not necessary to actually learn the truth.
"I was forced to smuggle it in as a religion—which it is not—and con the marks into tasting it by appealing to their curiosity" (36.139)
Mike notes that his own religion is not a religion at all but a way of viewing the world, humanity, and one's place in each. Do you think Heinlein is suggesting this role about all religions?
Mike grinned with unashamed cheerfulness. "I am God. Thou art God… and any jerk I remove is God too." (36.153-54)
First, this is hilarious. Second, we hear all the time about scientists "playing God," but here Mike suggests that such acts aren't so much "playing" as they are "being" God. And since, according to Mike, we are all God, there is no issue; it's only natural. Cue Joan Osborne.
"A very few, just these few here with us, our brothers, understood me and accepted the bitter along with the sweet, stood up and drank it—grokked it. The others, hundreds and thousands of others, either insisted on treating it as a prize without a contest—a 'conversion'—or ignored it. No matter what I said they insisted on thinking of God as something outside themselves." (36.166)
Here we bring the "Tradition and Customs" theme into the religious one. Mike runs into the problem that once someone is raised to consider God in a certain light, he is not very likely to change his mind.