"Rather than taking part in official Christianity with the thousandth part of my little-finger nail, I would rather engage in the following display of seriousness. A flag is purchased at a hardware store, it is unfurled; with great reverence I approach it, lift up three fingers and swear fidelity to the flag. Thereupon, rigged out in a cocked hat, a cartridge-belt and sword (all from the hardware store), I mount a hobbyhorse, proposing in union with others to make an attack upon the enemy, with contempt for the mortal danger into which I am evidently casting myself, with the seriousness of one who knows what it signifies to have sworn fidelity to the flag."
Kierkegaard's "Attack upon 'Christendom'" [277-78]
Here's the joke in "modern English":
Instead of joining the state's church, I propose instead the following. I will buy a flag, a uniform, a gun, and one of those battery-powered toy jeeps that little kids drive around their yards. Then, with grave seriousness, I will declare my faith in God's protection, furl out my flag, and call for the members of the state's church to follow me into battle. I will then hop in my toy jeep and begin driving toward the battle field.
The deeper point being made isn't possible to ascertain from just this snippet; you'd have to put the joke back into its context and find the larger point being made in the text around the joke. (edit: but, see racer-v's explanation)