Bergman says it too. Believing in God is same as believing in love. If you can believe in love, you

could rather believe in God as well. If by some rational explanation, God has to be completely ruled out, then by a similar reasoning there should be no reason for love either. A world without belief in God could be as dreadful to some as would be the world without any love, trust or friendship.
At the end of the movie, David asks his son, to stay calm and tells him that "God exists, because we all love Karin... Love is God! " We don't have anything to define our existence and so we have to pick up something and hold it to our heart. Whenever our holdings are gone, we pick something else up and hold that to ourselves. That is how we define ourselves - "Everything would be alright. Just keep faith in something".
I don't think anyone else can present existentialism better than the way Bergman does. He repeatedly makes me think that cinema is the best, most true, and most efficient form of art known to humanity.
Again, like in many other Bergman movies, I might not have understood all the connotations in the movie, including the title - Through a glass darkly, which is from a Biblical verse. In many of the dialogs there could be (and I am sure there were) allusions to Bible and some other writings, like a few by Dostoyevsky. Unfortunately, I am not too much aware of the literature that Bergman was expecting his viewers to be building the thoughts upon.
Nevertheless, the movie was terrific. It went boring for a while, especially the part after when the fact of protagonist's illness was known. But in around 20 minutes, the story picked up again and climaxed magnificently.
The performance by the protagonist was excellent. I don't know if anyone else could have done such a job. Harriet Anderson (Karin, in movie) suits very well to the character. Gunnar Bjornstrand played his role of the detached writer just perfectly.
The climax is magnificent, it rolls the whole movie into a few dialogs. All the chaos of the movie- Karin's pain, Minus's troubled juvenility and David's disconnection from it all, everything seems to survive just through faith - for faith
is indeed such a powerful thing - the central theme of Bergman's movies.