The Bat that flits at close of eve Has left the brain that won't believe. The Owl that calls upon the Night Speaks the Unbeliever's fright. (25-26)
Blake uses a bat and an owl as symbols for the minds and thoughts of unbelievers because they're nocturnal creatures. They're stuck in the darkness of the material world and don't have access to the light of the spirit or of imagination.
A truth that's told with bad intent Beats all the Lies you can invent. (53-54)
It's worse to use a truth for false purposes than it is to just tell a lie. Not only does it cause harm, it corrupts the truth in the process. On the other hand, you might tell a lie for a basically positive reason.
He who replies to words of Doubt Doth put the Light of Knowledge out. (95-96)
For Blake, there's no point in arguing with someone who's a professional doubter, since they don't really want to be convinced. You'll just end up doubting your own convictions if you fall into this kind of argument.
A riddle or the cricket's cry Is to doubt a fit reply. (103-104)
Blake suggests that, instead of getting into an unproductive argument with someone who's not interested in being convinced, you might as well drop a riddle on your interrogator.
The emmet's inch and eagle's mile Make lame philosophy to smile. (105-106)
Since the emmet (or ant) is close to the ground and the eagle's far above it, Blake's saying that discussing the limits of perception is something that's making philosophy feel a little overconfident. (After all, philosophy itself can't lead to new perceptions, but can only reason about preexisting ones—that's why it's "lame.")
If the Sun & Moon should doubt They'd immediately Go out. (109-110)
The sun and moon are too busy doing their thing—radiating and, in the moon's case, reflecting light—to be distracted by a lot of trivial doubts that don't lead anywhere. Maybe, implies Blake, humans should be doing the same?