I would’ve let the blasphemer have it right in the middle of his diatribe.
I did that in the 90s in a Methodist Church where I had been a board member during a "let's all sing kumbaya about gay relationships and gay pastors" sermon — stood up and rebuked, quoting the Bible, and walked out — but no one went with me, then or in the months to follow. I got a list of sins with mine highlighted in yellow marker mailed to me by the pastors, and one phone call from a member who would miss my hard work on her committee, but that was that.