"I'm not really religious, but I do believe in God."
I've heard this sort of thing from many people, but I'm not entirely sure what it means; or rather, I'm not sure what it means for them.
When someone says, "I'm not religious," they are saying that they do not hold themselves bound by any particular set of dogmas, ritual obligations, or ethical principles that are rooted in any kind of divine revelation. (I think that's a fair way to put it.) They wouldn't consider themselves Catholic or Methodist or Non-Denominational* or Buddhist or Muslim or Sikh or Hindu or anything else one could capitalize.
*(Yes, I know, "non-denominational" Christians are not exactly an organized group, but they have so multiplied and seem to share so many traits, they really have become an identifiable subset of Christianity.)
So, these folks do not believe in any set of religious beliefs or specific divine revelation. Yet they will then profess that they "believe in God." What does this mean? If this belief in God does not include any belief in anything God may have revealed about Himself, what is left for this "belief in God"? Only the barest minimum.
When people say "I believe in God" in this way, what they mean is: "I assent to the intellectual proposition that 'God exists.'"
This, to my mind, prompts all sorts of questions: who or what is this God whose existence you affirm? How did you come to know God? What do you know about God? Have you met God in some way? Or is God not a "meet-able" thing? That is: when you say "God," what are you referring to? I don't know how far simply affirming God's existence can get you in addressing these questions.
When Christians say "I believe in God" (as Catholics do every Sunday when they pray the Creed), they mean much more than "I assent to the proposition that God exists." They mean something more akin to what is meant when a wife says to her husband: "I believe in you." She isn't just saying, "I affirm that you exist." She is saying: "I trust you. I have confidence in you. I know you and I know what you're about and I know what you can do, and I know that you will do what needs to be done." There's believing, and there's believing.
I sometimes get the impression that some people have the impression that God sits upon his heavenly throne with a huge ledger in his hand, ticking off boxes for each one of us to see if we meet the bare minimum requirements to not merit being cast into the fiery pit of eternal despair, and that the barest of minimums is "Acknowledge my existence," as if God were the geeky kid at school who would let you come swim in his pool if you only would say hi to him. But God wants more from us than a passing greeting in the hallway, and God wants to give more to us than an afternoon pool party, and we need more for our fulfillment than sunshine and chlorinated water.
We were made for communion with God, loving friendship, a participation in God's own life through his gift of grace--to be "partakers in the divine nature" (2 Peter 1:4). This participation is brought about through our union with Jesus Christ, the only Son of God, through whom we become adopted sons of God. In Baptism we die and rise with Him; through Confirmation we are sealed with His Spirit; through the Eucharist we are fed with his Body and Blood and filled with his grace as the sap of the vine fills the branches. That's what we aim for. Not just nod of the head to an acquaintance, but the embrace of a lover.