The first thing we did was go back over the definitions of atheism and agnosticism. It's admittedly a somewhat difficult concept to grasp at first, as is the difference between "I believe there is no God" and "I lack belief in God." The distinction ultimately lies in the idea of burden of proof: it's the theist who's making a claim that something exists, and the atheist who's holding out for sufficient evidence of that claim. Eventually I explained it using a very simple analogy: Imagine a thermometer with a notch two-thirds of the way up labeled "God exists." For the atheist, the fluid in the thermometer (the evidence for the claim) has not reached that notch (the threshold for rational belief). That seemed to work well, so I'll probably be using that comparison in the future.
We also talked at length about miracles. He described an event a few decades ago in which, when driving at night, he changed lanes on a whim—just in time to avoid a parked car that he hadn't seen. He suggested (while not putting too much stock in it himself) that this could have been divine intervention. I pointed out that we tend to disproportionately remember extraordinary events, and introduced him to Littlewood's law—the idea that given the sheer number of small events that take place in our lives, we should expect "miracles" at a rate of roughly once a month. I didn't go into the general unreliability of memory; I'll save that for another time.
He also asked about spontaneous remission of cancer following prayer, to which I pointed out that such remission occasionally takes place whether or not people are praying. He'd heard about some crying Catholic statues as well. In response I brought up the Hindu milk miracle: In 1995, hundreds of believers in India and abroad witnessed statues of Ganesha "drink" milk that was fed to them with a spoon. Neither of us believe in a miracle that took place in modern times, with countless verifiable eyewitnesses and even video evidence, so it's only natural not to believe in lesser miracles either.
You may not be able to prove God, but you can't prove love exists, either, even if you can measure the chemicals.
—Love is by definition a feeling, so the very experience of love shows that it exists.
But couldn't the same be said of God?
—No, because God is supposed to be an autonomous agent who acts independently of our subjective experience.There were a grab bag of other subjects. He asked about Old Testament prophecies predicting details of Jesus' life. I responded that some details were probably invented by the gospel writers after the fact, and some of the "prophecies" weren't even meant to be prophecies in the first place. He was under the impression that the eye couldn't have evolved, when in fact we have a detailed understanding of how it could have come about. He also found it absurd that matter could have created itself. I haven't studied the details, I said, but we don't really know what happened before the big bang. It may be that matter has always existed, or that it's not even useful to talk about a "before" in the traditional sense (like asking what's north of the North Pole).
The whole discussion was quite genial, and although I don't know that my dad got any nearer to my position, I think it was a success. He even said that in a strange way, these discussions brought us closer together, and that's more than I ever could have hoped for.