James 3:11 in context:
9With the tongue we praise our Lord and Father, and with it we curse human beings, who have been made in God’s likeness.
10Out of the same mouth come praise and cursing. My brothers and sisters, this should not be.
11Can both fresh water and salt water flow from the same spring?
12My brothers and sisters, can a fig tree bear olives, or a grapevine bear figs? Neither can a salt spring produce fresh water.
This discusses the (claimed) saved, not atheists. It is also talking about the saved being two-faced, mixing good and bad.
You would understand sooo much more if you understood context and the big picture.