I don't know, but I presume he meant something about man's place in a cosmos designed by a personal God, and whether man is special and unique in all the universe.
On the face of it, as a Christian myself, I once believed talk of extraterrestrial life was BS because the Bible doesn't mention it... but...
Then I realized a couple things... a) yes, the Bible does mention another created class besides humans, specifically the angels, and b) just because the Bible doesn't mention something doesn't mean it didn't happen.
I now think that just because the Bible was silent on whether God created other beings anywhere else in this vast universe, that doesn't mean he didn't. On the other hand, all of creation is fallen because of original sin. And on the other other hand (or... on the gripping hand), why rule out the concept of a multiverse? Maybe there's an alternate universe where another Adam and Eve remained sinless and live in the perfect creation as intended? Or others that also fell, or maybe some where they fell but angels didn't rebel.
I'm no theologian (as it should be obvious) and much of my thinking is purely speculative and believe me... I haven't fully fleshed out these ideas. In the end, to be honest, I don't care... maybe aliens exist, maybe not... it wouldn't shake my faith either way. But some people, it would affect them in profound ways, shaking their belief system to the core.
Nicely done with excellent points.