1 Thessalonians 1:9–10 might suggest to us that a Christian is one who has turned to God from idols to serve the living and true God, and to wait for His Son from Heaven, whom He raised from the dead. It's striking that the Thessalonian description of Christianity puts waiting for the Son of God from Heaven on par with serving God. I don't mean that those two things are equivalent, but that they are of equal importance.
Philippians 3:17–21 says something similar: we are to model ourselves after those who walk like the Apostles, who are waiting for the Son of God to come from Heaven, to change our bodies to be like His. I'm not sure I've ever really obeyed that exhortation: I'm not sure I've chosen role models based on whether they're waiting for the Son of God from Heaven. But that seems to be what the passage is telling us to do.
I'm old enough to remember a time when the imminent return of Christ was a common belief among evangelicals. When I was growing up in the 70s and 80s, it wasn't a very controversial idea. Somewhere along the way, that seems to have fallen out of favor.
There are many godly believers who hold different views to mine about the Lord's return. I'm not very dogmatic about eschatology, because I've lived long enough to have been proven wrong about a great many things. But I do insist that the epistles present the Lord's return as something that can happen at any moment, and that the Apostles expected Him to return in their own time. 1 Thessalonians 4:15–18 make it very clear: Paul includes himself in "we who are alive and remain." 1 Corinthians 15:51–52 make it clear that Paul didn't consider himself one who must necessarily die.
I recall reading J. N. Darby, where he pointed out that it's when the servant says "my lord delays" (Luke 12:45–46) that he begins to abuse his position. The servant doesn't say the lord isn't returning at all, merely that he's not returning soon.
I confess that I have fallen into that trap. I haven't ever denied that the Lord is returning, but I have certainly acted like He isn't returning soon. And I am more and more realizing that with this subtle shift in thinking, I have fallen into all sorts of traps.
So I've been reminding myself that the Apostles expected the Lord to return in their own lifetimes, and I am by no means wiser than they. I've been reminding myself that this life is real and significant and what I do here matters, but I have been called to wait for the Son of God from Heaven.
Those two principles are in a bit of opposition to one another, and we've seen what happens when either one is ignored. On the one hand, those who forget this life is real and significant have a tendency to Gnosticism. On the other hand, those who forget that we're called to wait like the Son of God is just around the corner have a tendency to live like they don't think He's ever coming.
So let's don't fall into either error. Let's not live like this life doesn't matter, but let's not live like it'll matter tomorrow. It might not.