Jesus is said to have said that the mustard, the tiniest of seeds, grows into the largest of trees, where birds rest. The mustard seed is about one twentieth of an inch across, but seeds of many other plants are much tinier. The seeds of the prairie beebalm are about a thirtieth of an inch long and seeds of the prairie shooting star are about a fiftieth of an inch. Mustard is not a tree. It is not even woody. It is not even a perennial. The two forms of edible mustard are annuals and about three feet tall. The bur oak tree of the prairie grows to fifty feet on sandy hillsides with less rich soil and to ninety feet on prairies with thick deep loamy soil. As for the birds, small songbirds may stop briefly to pick at the mustard seeds but the plant is not the sort in which a bird is likely to stop to perch for a rest. In your spice drawer, the common celery seed and the poppy seed are considerably smaller than the mustard seed, and the nutmeg does indeed grow on a tree that grows to about 30 feet tall. But, sorry, mustard isn't the tiniest seed, it does not grow into the tallest tree, and birds do not rest there.
This little parable is repeated in Matthew, Mark and Luke. If that repetition proves Jesus really said it, well, Jesus was a terrible botanist. If the authors got it wrong, because one wrote it wrong and the others copied, one must wonder what else they got wrong. In the end, you might say that whether you have faith in a 'god' has little relation to the accuracy of the book that your belief system is based on. That might be true insofar as faith is concerned. But insofar as specific truths about our world are concerned, one has to wonder how much the book can be trusted.
If you are using your religion's book to be mean to other people, to discriminate against them because they believe in another religion, or to deny them rights because of something your book seems to say about them, you should realize you are on pretty shaky ground. Either your Jesus was wrong about every thing about the mustard plant and its seed, or the people who wrote about Jesus were wrong to say he said that wrong stuff. So the parts of that book that you use to allow you to be mean to people might be wrong too. If you use your religion to be mean to people, you are wrong. Jesus was a terrible botanist, or the authors of the bible were terrible biographers, terrible historians. The information contained in the mustard seed parable is simply not factual. Put another way, it is in error. Put another way, it cannot be taken literally. Did Jesus get it wrong or did his biographers get it wrong? Either way, if you are using things in that book to justify your meanness to other people today, what does that say about you?