What you wish to share and what you wish to refrain from saying is not the primary concern expressed by "Dans la Lune." He wasn't referring to your personal wishes, but to your best interpretation of Christian theology.
If I am not mistaken, there are repeated injunctions in the New Testament that sins are to be forgiven through Christ. It does sound very much like an ultimatum: either accept the theological belief that Christ died on the cross for our sins or be cast into the damned. However, I don't remember enough about the scripture to be able to say if this is an interpretation, or if it is litteral -- meaning there's something literally saying that the unbelievers will be damned. That would be closer to the objection raised by "Dans la Lune."
On the other hand, if it is what the text is saying, it conflicts with a core message of Genesis. God promised Abraham that all the nations shall be blessed through him and likewise to Isaac and Jacob. It's clear from this that not everyone was to adopt the same rituals and theological beliefs (i.e., become a Jew in this instance). It's also clear from how God acted against the whole world, Sodom, Gomorrah and, later Egypt in Exodus, that the thing he cares about is how people behave. Some laws were issued only for Jews and, others, for all mankind. Being a Christian is a matter of theology (how you answer certain questions regarding the New Testament), but pleasing God is a matter of behavior. Of all the laws listed in the first five books, only the command to not covet concerns a sin of thought. What you actually do of your own accord is therefore of paramount importance as it should, insofar as God is indeed just. This accords with my impression that either what the Bible says is relevant to all of humankind, or it is irrelevant.
Moreover, notice that God's existence is never questioned. It is always assumed. The problem of someone who might doubt that God exists doesn't seem to be a concern. Faith and belief are mentioned in scripture, but in most instances, I can call to memory, it makes no sense to read this in the modern sense of "belief that God exists." It always makes sense to read it as "having trust in God." When something is important, you repeat it. Either the idea of a godless universe was thought to be so silly that it was not worth addressing, but that raises a problem for anyone who is intellectually honest and believes the Bible was divinely revealed: manifestly, many very smart people do not find the question so easy to answer. Or, that wasn't the important part of the book.
To round up this thought, I like how Jordan Peterson answered this question. His first answer was that he acts as though God is real. His second answer is that if someone was truly convinced of the truth of biblical scripture, they should be deeply concerned not to step out of line. So much so that it would take great courage to claim they believe in God, lest God smites them on sight for laying claims to beliefs to which they do not fully commit. There is yet another way in which you can understand what believing in God means, one which I peculiarly enjoy as an economist: what you believe is what you actually do. The rest is hot air.