I had a similar discussion with a family member, about the existence of the Christian god, where I received that exact response. My wife was sitting right there. I responded with something along the lines of, "True, but my 'faith' in her love is already backed up by evidence, and besides, I have plenty of evidence that she exists. If there was evidence for God and evidence of His love, I would happily put faith in that too."

But I agree - it definitely caused me to pause to consider a tactful response.