I think that many assume that once a God is acknowledged that religion must follow. Personally, I do not find that to be factual. One may believe in God without necessarily joining others, even those of like beliefs.
So how do you form your basis for your belief in God? If you pondered how the world came to be, where it all came from, etc, you could determine that their must be some being not of this universe - the Creator, the first mover, etc, however you want to define it. The question is, then, without religion and revelation, how do you know anything about God? Through private revelation, or mere logical conjecture.
And to address the OP, I think you could bring philosophy into the argument. If there is a God, and God created the Earth, it is evident that God created man (which, so as not to through people off, I mean ‘created’ as ‘caused man to be’, thus avoiding the Creation / evolution debate to an agnostic) to be the highest order of creation.
So why were were created as the highest order of beings on Earth? The only ones with our rational intellect and free will. Clearly it was not to be slaves to God - our free will allows us to get around that. Are we only for his amusement? Look at any religion and God (or the gods) are typically involved in the matters of humanity - sometimes to save, sometimes to damn. Therefore, we can conclude that God cares about us - that God loves us.
So what does one do when they are in love with someone? They desire to be loved in return. It strengthens that love between them. So if God created us out of love, and wants to be loved in return, then surely God would instruct us as to how we can return that love. If not, it’d be like giving a bunch of toddlers a deconstructed motor and say “Make it run.” There would be lots of guessing, and maybe (out of pure luck) get some things right, but for the most part, be completely wrong.
Now, you have to luck at what religions claim revelation from God, and how those revelations tie in with the idea that we were created out of love and with free will. As to this point, CS Lewis advises to look towards Hinduism or Christianity, as Hinduism is (apparently, to him) the most logical non-monotheistic religion (and I know the complexities, so I hate to ‘label’ it, but hopefully it is accurate enough for this discussion), and Christianity is the fulfillment of Judaism. Concerning Islam, I refer again to Lewis, who calls it the greatest Christian heresy.
Personally, I find monotheism to make the most sense. If there were a multitude of gods, there would still have to be one originator. We see in Greek mythology that Athena grew out of the head of Zeus, while Aphrodite was birthed by the sea. And with other polytheistic religions, you find ‘good’ gods and ‘evil’ gods, or at least gods that ‘war’ with each other (whether directly or through the manipulation of humans), and I never found that to work with the notion of a loving God.
Thus, we settle on Christianity. That in itself is no small task, but I would ask that you examine those who still claim their ties back to the Apostles. Some of the Reformation churches do claim valid orders, as do the Orthodox, and obviously Catholicism, but a lot of ‘Bible’ churches teach the other churches were wrong. We see in the Old Testament that God instructs His people when they go astray, but these churches believe that God allowed His followers to stay in error from shortly after Christ’s death (the ECF’s are clearly in line with the current teachings of the Catholic Church) until the Reformation (or even today, with some of these mega-churches?
If you can whittle down to those claiming back to the apostles, you look at the Sacraments, the words of Jesus, and the authority given to Peter as Prince of the Apostles. It should be pretty easy from there to sign up for RCIA and start the process!
