In philosophy, there is a distinction made between knowledge gained a priori and a posteriori. As the phrases themselves suggest, knowledge gained a priori is knowledge that you have prior to any experience in the world. For example, you can know that two and two make four without adding up two and two of everything that exists to verify. Knowledge gained a posteriori, however, is knowledge that you have after experience. An example of a posteriori knowledge is knowledge of whether or not there is yogurt in the refrigerator. You couldn't know such a thing simply by sitting in your armchair and thinking long and hard about it; you have to go and see for yourself if it is true.
Now, this may seem abstract and unimportant for a lot of people, but actually this distinction is very important as regards theological method. The question arises: do we know about God a priori, or a posteriori? Consider it phrased another way: can I know about God and what he's like without his revealing it to me, or does he have to tell me what he's like for me to know him?
Certainly it seems we have good reason to think knowledge of God must come a posteriori. Consider these biblical texts which suggest that knowledge of God must be had on the basis of revelation:
All things have been handed over to me by my Father; and no one knows the Son except the Father, and no one knows the Father except the Son and anyone to whom the Son chooses to reveal him (Mt 11.27).
No one has ever seen God. It is God the only Son, who is close to the Father’s heart, who has made him known (John 1.18).
These passage suggest strongly that humans can only gain knowledge about God if God reveals it to them in some way. No one knows the Father except the Son, and the Son has to choose to reveal the Father if others are going to know about him. And when John says that no one has ever seen God, the suggestion is obviously that people lack knowledge about the Father's heart, about the true character and interests of God. The corollary of this is the affirmation of the apostles that Christ has made God known to the world:
[God] has made known to us the mystery of his will, according to his good pleasure that he set forth in Christ, as a plan for the fullness of time, to gather up all things in him, things in heaven and things on earth (Eph 1.9-10).
It may not immediately seem obvious, but this important truth has tremendous consequences for the way we think about God and about the Christian life. If we start with this a priori method, we run the risk of turning God into someone who is entirely like ourselves: someone who agrees with everything we do, everything we believe, everything we feel, etc. But if we start with an a posteriori method, and determine that God has to reveal to us what the truth about himself is, then we might find ourselves quite surprised at what God has to say!
This is nowhere more important than as regards the infamous problem of evil. This is the question on the minds of so many people in the world: if God is so good, why is there such suffering and evil in the world? Either God is not good after all, or else he's too weak to prevent all this evil, or else he doesn't exist at all. But what can we say in response to this argument?
Notice that the argument assumes an a priori method about God and about what concerns God should have. The person who offers this argument thinks that she knows best what God should be doing in the world, if he truly exists and if he is all-good and all-powerful. But there is no reason to think that we can have a priori knowledge about that at all! And as the biblical texts I cited above suggest, our knowledge of God and his purposes must come a posteriori -- he has to reveal it to us.
Now what do we do in the face of this suggestion? Should people double down on their own a priori method and assert dogmatically: No, not sin but suffering is the problem! How can you know that? How can you insist that you are right and God is wrong? Might it be that insisting on suffering as the problem is the easier way out, because you can turn yourself into a victim who gets to point the finger at God, whereas taking sin as the problem means accepting you are the wrongdoer and to blame?