You don’t need a source if it’s God’s truth. God’s truth is independent of whether you could convince someone of it or not. All good people who haven’t done bad things (without repenting) know God’s truth in their heart, thanks to the Holy Spirit and God’s grace. As such, I could tell you’re not a believer by asking that question as believers just nod their head to it since they all know the answer already. I don’t owe unbelievers anything once I’ve determined their lack of faith in God. Godspeed.