Aside from the evidence that worship works, and the evidence that you can experience God, two other convincing evidences for the existence of God and the reality of the spiritual struggle have made themselves evident in my life this week.
The first is the dark valley experience. This always happens after a 'spiritual high' - a mountaintop experience. I was touched by the Spirit of God at church on Sunday night. I knew his presence, I felt his power. And so it was that on Monday morning I found my thoughts drawn to all sorts of inappropriate subjects and somehow the least godly CD in my record collection ('Tenacious D' if you must know) found its way to the top of the heap and got listened to; so my mind has been re-playing songs with more than a liberal sprinkling of profanity and some very dubious subject matter over the past few days...
This always happens - after the mountaintop comes the dark valley. If there were no spiritual struggle this wouldn't happen, at least not all the time. But every time I have a spiritual high, the temptations come thicker and faster than usual. I know I don't fear the devil, or think that he is as powerful as some would have it, but he is the father of lies and is very, very good at putting thoughts in your head when you're not looking. This may be his only power, but it is still a power.
The second effect is more or less the same. Prayer. It should be easy - just talking to God. But it isn't. Distractions and distracting thoughts come at you thick and fast when you try to pray. There he is again, the father of lies. In fact, even if there is no God, talking to empty air should be easy. The only reason that it isn't is because someone is out to prevent you doing it.
So there you have it, two negative evidences of the reality of spiritual things to go with the positive evidences I've discussed before.