Rule 10 of the spiritual exercises says that when experiencing spiritual consolation you should buckle up for upcoming desolation. Ok- 1st thing: what's the difference between consolation and desolation? Well, to make it really simple, consolation is like heaven on earth and desolation is, well, the opposite. Of course, this pertains not to how things are at work, at home, in the stock market, with your favorite sports team...but to your spiritual life and your relationship with God. Sometimes we can feel like St. Peter, like we are walking on water. Then, all of a sudden, it can feel like God does not even exist, like he totally fled the scene and is nowhere to be found. What often happens is that we then live accordingly, either as if God did or did not exist. Which means how we live fluctuates like crazy. Good or bad, in light or shadows, before the Almighty or as if I am the almighty.
St. Ignatius says in this rule that when I am experiencing spiritual consolation- i.e. when I believe in God and it feels good and I am happy and loving rather than sinning- I should not just sit back and enjoy. Yes, enjoy it, but without the "sitting back" part. I ought to be preparing myself mentally, aware of the fact that this spiritual high is not going to last forever. On the contrary, a spiritual low is coming. For sure. And there will be a high again after that, but in the meantime I need to get ready for the time of trial. What does that mean? It means that even when it feels like hell on earth, I will live as though God exists and I will strive to love him.