When you're speaking with an atheist, the Bible says you are dealing with a "fool" (see Psalm 14:1). This isn't someone who is a clown or a court jester. They are people of skill. You're not dealing with someone who is of a low intellect. That person can't help having a low IQ.

You are dealing with someone who has willfully blown out the inner light that God has given to every man. Such a person has already disqualified themselves from reasonable conversation. They can't be reasoned with because their worldview denies that which is axiomatic or commonsense. This is because atheism gives him the temporary license to look at porn, to fornicate, etc., without a sense of moral responsibility to a Creator. For a sin-loving sinner, it's like a dream come true, and so he will say anything to defend those pleasures, including denying that which is obvious.

What you must do is move away from his "carnal" mind that's at enmity with the moral Law (see Romans 8:7), and the place to go is where Jesus went--to his conscience.