Do Christians have faith in oxygen/air?

[RoadRunner79]

Okay, so now I'd distanced myself from the church for a few months now.

Three months ago, my partner asked me if I had faith in the air we breath.

Of course my answer was no, because it is obviously and irrefutably there.

You could guess what that person asked next! 

Next question was: "So why do you have faith in God?"

Thinking about that question for several weeks now, and I'm having a strange feeling. 

Am I overthinking because I can't think of a strong answer? What am I missing?

I think that your problem with the question is that it is an equivocation of the word faith.  The word "faith" can be used in couple of different ways, and some I think particular to Christianity.  To use a sense of the word, which is particular to the religion, and compare it to another sense of the word used in a different way, is not correct thinking?

Do you have faith in science?  Do you have faith in your partner?  Do you have faith in Donald Trump?
None of these in answering yes or no; seem to have anything to do with belief that the object of the declaration is irrefutably there.  Would you agree?

Frankly, I think that question of do you have faith in air; is a rather odd to begin with.  My response would be faith that it will do what?   (thinking that it will not suddenly leave the space I'm in, or quit working, the way that it has always done.  So I don't even think the first question makes sense.