Well, yeah, like I said, this is pure conjecture. It does, however, remind me of that book that was published some time ago(I was still a kid in middle school and a devout christian the first time I saw it)Letters To God, and all those letters that were supposedly 'mailed' as well most likely met similar fates.
I think if I had to draw any kind of conclusion from what we can tell by the picture, better discretion would be appropriate in discarding a letter of this nature, considering what it looks like. I mean, it's a letter to god in the trash that was noticeable enough for someone to pick it out, take a picture of it, and post it on reddit. In my eyes, if I were Marty, and I saw this(however unlikely that is to happen), I would be awestruck, and I'm sure my perception of god would be tainted, and my parents would owe me an explanation.
I understand there is no mailing address for god, but still...discretion.