Steve, you asked:

What if there really was a God? Would I then be in an awful lot of trouble?

That depends whether the "a God" was really all-knowing, to be
aware of your unbelief.
    and
Whether the "a God" gives a durn whether you believe in him/her/it.
    and
Whether the "a God" has and chooses to exercise the power
to punish you, putting you in an awful lot of trouble.

The putative existence of the generic "a God" does not necessarily entail positive answers to any or all of the above.

Dave Oesterreich