X-Files Book Club

I remembered the paragraph surrounding it as being much more desperate-feeling so the phrase is probably a lot less significant than I'd imagined.

No, you're right, infinitlight. We first hear of it in How gravity Works, but then Mulder tells us again, in more detail, in Home From the War, when he's telling Scully what his memory of their first time together was like. That's when the full impact of the scene really comes through:

"It was after Modell," he began hoarsely. "We stayed in Virginia
that night, we were both just fried. We checked into this crappy
hotel, we got a double, just figured we'd crash for a few hours and
drive back early in the morning. But we couldn't sleep, we were
talking and you, uh, you went to get us a couple of sodas or
something. I lost it, I just curled up in a ball, I was crying
and... how I'd almost killed you. And you, ah," he motioned
abstractly with his hands, "you lay down with me on the bed for a
while, until I got hold of myself. I went and took a shower, and I
heard the door open, the bathroom door. It was one of those
glass-door showers, hazy, and I could see you through it. You were
naked, I could just see the colors. You were beautiful, Scully, you
were so beautiful I was speechless. You got in with me and you said
'we're alive.'"