What does "writ respectable" mean?

Writ is an older form of "written".

We still use it in the phrase "writ large"

"Writ respectable" means something like "expressed in a respectable/ acceptable manner".

I should have mentioned the use of "but" in your example.

Again this is an outdated usage- here it means "nothing but" or "nothing other than".

Nowadays we might substitute "simply".

"agnostic was but atheist writ respectable..." Roughly, it means that the word "agnostic" was used instead of "atheist" but only because the speaker wanted to sound more respectable. ("Agnostic" is a softer, more 'respectable' position than "atheistic.")

"...and atheist was only agnostic writ aggressive." Here, it means that the word "atheist" was used instead of "agnostic" probably because the speaker wanted to sound more aggressive.

In that quote, the real message is that there's no real difference between atheism and agnosticism. People choose 'agnostic' when they want to sound softer, but use 'atheist' when they want to be more aggressive, even though their real position is the same in both cases.

Excellent usage! It sounds with an atmosphere of dogged gentility.
Thank you both.

Why do you figure the prescriptivists haven't been all over this one, McTag? Isn't 'written' an abomination that must be stricken from the English language?

I might regret saying this but...I'm not sure what you mean.

"Written" is a standard word in everyday use, is it not. Would you care to expand and elucidate your point?

I was just joking, McTag, and checking to see if you were still alive. I hope you are hale and hearty.

He's bathed in the halo of life, I believe.

Too right. Thank you, gentlemen, for your kind concern.