The song "Imagine" is usually misunderstood by religious people. When John proposes us to imagine that there's no heaven or hell he's not saying that they don't exist (beyond his own belief), he's saying that we shouldn't be moved by an after-death prize or punishment, we should be good to our brothers because that's good for itself, and good actions bring a prize on their own. When John imagines no religion he's talking about the source of fear and war, not about its message, which is actually the same message he's preaching in this song. John is just telling us to live for today because we should try to make heaven on earth, even though that's not possible, in part because of that fake religious people that do the opposite of what religion says. In fact, the message of "Imagine" is much more Christian than what many self-called Christians preach with their acts.
"Now the fruit of righteousness is sown in peace by those who make peace." (James 3:18)