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Even the most devout people, sometimes have doubts. That's why it's called having "faith", and not called having "proof".
Just today in my class as I was teaching about the Sunday of Orthodoxy, an 11 year old girl very seriously asked me how do I know for sure that God exists. I was thrown for a loop. How to handle this?
How to explain this to class of children. I simply know. I feel Him everywhere. I see Him in everything....but, this won't convince a child.
I explained very succinctly, that the prophets had informed us that a Savior would come. Christ met all the prerequisites. With His life Christ showed us God. His generosity, His love, His caring, not to mention the miracle of His resurrection. He was heard and seen by His disciples, who also saw Him physically ascend to the heavens. They have told us this, and the knowledge of Him has passed on through them, through the saints, etc. to us.
She didn't buy it. She still whined "but...how do you know for sure? What if they had all hallucinated everything?" That's kids for you.
Knowing that her father's parents had passed away before she was born, I asked her if she knew her grandparents. She said no, they died before she was born, but, she knows their names, and she's got a picture of them on the wall at home. I then asked her if she had actually seen them ever with her own eyes. She said no...and was irritated with me....because she already told me they died before she was born.
I replied..."well, if you haven't seen them with your own eyes, how do you know they were real?"
I didn't convince her God was real, but, I made her stop and think.