Every Christmas, Charlie Brown gets sad and can't figure out why - he knows he should be pumped for Christmas like all the other kids, but he's depressed. Maybe it's the commercialism (his theory). Maybe it's the fact that he's got no parents and is raising his little sister on his own and is completely bald (my theory). Or maybe it's that this sounds exactly like seasonal affective disorder (a common mood disorder, typically affecting people at the onset of winter, in response to colder weather and less sunlight, amongst other causes).
Of course, this leads to all the kids treating Charlie Brown like garbage for daring to be clinically depressed. Lucy, Violet, and even Linus give Charlie Brown endless hassling for not feeling great. Lay off, you little assholes, he's like 8 years old and is still having to deal with nearly getting strangled by his dog.
Charlie Brown is in a rough spot - everyone's pissed at him for getting "the wrong tree" and he just wants someone to explain to him the meaning of Christmas. And Linus, his best friend in the world and the person he trusts above all others, goes and delivers a freakin' sermon.
Yes, Linus, that's the origin of Christmas, but it sure as hell isn't the MEANING. All Charlie Brown really wanted (and more importantly NEEDED) to hear is that Christmas is about giving, and friendship, and family, and togetherness, and NOT commercialism. All you did was recite some Bible verses, without any context or insight. And predictably, Charlie Brown gets basically nothing out of Linus' speech, and just leaves to go try to decorate his twig-tree.
But here's the real thing: just stop giving him shit about picking "a bad tree" and realize that - since a tree is just a symbol - picking the most humble and modest one is actually MORE IN LINE with your "true meaning of Christmas" than any of the others. That's kind of the point of your Biblical speech - your savior had the most humble and unassuming birth for the supposed son of God: he was born in a manger, surrounding by livestock, and with no fanfare around it.
That's how you know there was no meaning behind Linus' sermon - even he doesn't get it.
Oh yeah also don't forget Linus prays to the Great Pumpkin for a good portion of the year - and praying to false gods is a pretty big no-no in the Bible, so probably don't take him too seriously.