This opening your heart up I think is an important point. People do seek an answer... "God are you real?" and people do find an answer? Not necessarily. They find answers. Different people asking the same question find different answer in different religions or spiritual paths.
Once a belief or path is found, they can keep "opening" their heart to the teachings and wisdom that they are learning. And I have no reason to doubt that what they find is the truth they were seeking. But that truth that they've found isn't necessarily the same or has to be the same someone else has found. And since some spiritual paths have many Gods, no God, one God, or one God but in three parts... I don't think it is possible for spiritual/religious people to say that their beliefs are the correct ones.
One God is the belief of some, and they took that belief and followed the teachings and practices of their religion and have gotten to a higher spiritual level. But so have people that have believed something totally different. That is why I think it's not dependent on what a person believes, but that they believe it, trust that it is true, and follow it. If that belief started with one God, then I'm sure they will become more and more "one" with that God.
But if that "God" isn't provable, then what is it that they are believing in? An idea? A concept? A belief? And why is theirs right and some others wrong? Or even why is it wrong to even doubt some or all of these concepts of God? Because even some "believers" in God, do doubt the concepts of the Gods or God that are held by other religions.