When people ask me how I *know for certain* that there is a loving God in Heaven, I usually answer that I've not seen Him, but I've seen His work.
John 3:8 The wind bloweth where it listeth, and thou hearest the sound thereof, but canst not tell whence it cometh, and whither it goeth: so is every one that is born of the Spirit.
In other words, you can't see the wind, but you sure can see stuff blowing in it.
I understand the doubters. When my third child died as a baby, I had to go all the way back to "Is there a God?"
Yes, there is. More about Jessica in a
separate thread.
But I think what I point to the most is the first time I ever saw a preacher have to shut up and let God preach the sermon. We didn't see Him with our physical eyes, or hear Him with our physical ears, but we all knew that God's Holy Spirit was present in the room. The preacher said, "I had a sermon prepared, but I can't preach it." And he shut up. God took over.
After a silence during which God spoke to every individual heart, the pastor stepped to the microphone and counseled the congregation, "Don't be ashamed to cry in the presence of God." Well...I could do nothing else.

I for one was bawling my everloving head off. And I certainly wasn't alone.
It is hard to describe that experience. The apostle Paul said, "Spiritual things are spiritually discerned." However, I would answer a skeptic like this: If I had been alone, I may possibly concede that I might have been having some kind of delusion or emotional breakdown. But when a hundred people in the same room all describe the same thing, it is no delusion.