Seek the LORD while He may be found., page-22

"So, what is this process they have washed their robes and made them white in the blood of the Lamb. ?"

Your wording is questionable here.

They come out of and THEN wash their robes, so that means they went through the tribulation with dirty robes = holding untruths = filth.
They wash them with the truth, the truth of the REAL Lamb's blood.
Hence Jesus exposes these untruths 2 Thes 2:8 I think, BUT, he has already gathered his elect (the Few he ref), prior and then it's wrath time.

Hence, you are told to come out of her.