Hi, Charlemagne II,
Two observations to share:
1.) There have be atheists for a long time … King David even addressed these people Psalm 14:1-7!

With such an established track record… it is obvious that some ideas will simply not go away on their own.
One initial idea is review the logical arguments presented by Stl Thomas. Here is a link:
newadvent.org/summa/1002.htm
2.) This concern for not using a logical argument (done naturally in a civil manner) but rather to use love should not be viewed as a passive approach to the objective evil of denying God’s existence. Look at St. Paul who boldly (and charitably) argued with those who would deny the Cross of Christ. Look at the early Church Fathers as they combated the heresies of Arius and Nestorius. Moving along historically with have St. Dominic and St. Ignatius Loyola fighting heresies - with love and with cogent arguments.
In my opinion, to worry about these atheists “…hardening their hearts…” is not our issue - it is God who does the hardening or the softening as the individual case may be. Look at what St. Paul says in Romans 12:17-21. if we are to feed and give drink - we certainly should give a reasoned position for our belief.
**17Do not repay anyone evil for evil; be concerned for what is noble in the sight of all.o 18If possible, on your part, live at peace with all.19Beloved, do not look for revenge but leave room for the wrath; for it is written, “Vengeance is mine, I will repay, says the Lord.” 20Rather, “if your enemy is hungry, feed him; if he is thirsty, give him something to drink; for by so doing you will heap burning coals upon his head. 21Do not be conquered by evil but conquer evil with good. **
I submit that we are to present the best argument we are capable of - and, to do so in a loving way. We are to love the sinner and hate the sin - and telling the sinner why atheism is evil seems like a very charitalbe activity. If they leave with a hardened heart then it is something they will be responsible for when they meet God for judgment - nothing we can do can stop that. If they convert because of what we have said in a loving manner - then we have won a soul for Christ. I see no virtue in not acting so as to bring about the blessed conversion of a sinner because of the potential he
may dig a deeper hole for himself.
God bless
I think we have an obligation to answer atheists, but in the same way they attack us.
They demand proof that there is a God. We demand proof that there is no God. They can never be certain there is no God since that is impossible to prove.
Moreover, the real proof of God is in the love of God. So it’s true that you are not going to “win” a debate with an atheist, because the atheist has not learned to love God. He has not learned to love God because he has not opened up his heart to God. Don’t try to probe him too much in that direction. As david insists above, he will only harden his heart further.