I don't know what this was

"I think I understand it a whole lot better than you do."

It's been a while since I have been here, but I just had a theory and couldn't think of a better place to put it.

I don't think was only a kiss.

Just before the kiss, Aziraphale tells Crowley, "You don't understand what I am offering you."

To which Crowley says he does better than Az and proceeds to kiss. I feel he is somehow transferring a message/showing his memories to Aziraphale about his time in Heaven (or what he remembers from when he 'vaguely sauntered downwards', coded in the kiss, as–well, the Metatron and Muriel are already at the glass window– he doesn't want them to hear what he says to Aziraphale.

And I think THAT's why the first thing Aziraphale says is "I forgive you", to tell Crowley that whatever transpired is forgiven and he can come back to heaven.

And Aziraphale is also a tad heartbroken that it was just a coded conversation and not a genuine kiss instead, which is why I feel he feels his lips reminiscing and longing.

At that moment, he regrets saying yes to going to Heaven but can't escape it. I don't think there was a choice. It was Coffee or Death. Heaven or being wiped out of existence. So he immediately starts formulating a plan. [The long lingering glance in the direction of his diary*]

That minuscule flinch when Metatron tells about the Second Coming reminds Aziraphale of what Crowley had shared through the kiss. It is probably only playing out precisely as Crowley had predicted, hence the glance of silent acknowledgement at Crowley before he leaves.

And he is always has seen the 'niceness' in Crowley. That last look at Crowley is not Goodbye. Aziraphale is going to avenge Heaven for all it has done, including- in its own words, casting a prince of Heaven into the outer darkness. In fact, he can't wait to bring them all down; it is a frenzied smile that masks wrath and is thirsty for revenge.

*Me thinks Azi's also tucked away some of his memories inside the book (a recurring motif this season) while looking at the table before he leaves with the Metatron!

full of possibilities!