One of my favorites:

If people are good only because they fear punishment, and hope for reward, then we are a sorry lot indeed.

Albert Einstein

I would suggestion using some of the horrorable morals of the bible ..
Slavery, unfair treating of women as second class.murder., rape the list just continue and justificed by there holy book...
The bible is one of the most unmorale books ive ever read.
Christian don't have moral as in secular they have what you would call moral prenounment.

Also check the Divine command theory and Euthyphro dilemma.

This?

In addition to my previous comment, a quote that I often use (a quote from Free Wheelin' Franklyn, one of the Fabulous Furry Freak Brothers) when someone asks me if I truly believe what I say when I down play religion is "Is The Bear Catholic? Does The Pope Shit In The Woods".

You should just say - Explain the Crusades?
You know, the decimation of Byzantium in the early 13th Century, when Christians on their way to attacking muslims, decided to drop off at Byzantium instead. Destroying a 2000 year old civilization we knew as Rome.
Oh the Church apologised for that in 1996.

Ask them to explain the mass murder in the 16th/17th Century of women, who endured persecution from Witch-Finder Generals and a reassertion of patriarchal power, especially after the English civil war and Thirty Years War in Europe.

Ask them to explain slavery and how the Church in all its denominations paved the way for mercenaries and the state in the subjugation of millions. Which saw the murder and exploitation of millions. In the 1780's the life expectancy of a slave from Africa (if they survived the journey) was 3 weeks.

I don’t need a god to tell me the difference between right and wrong. I can figure that out out all by myself.

I far more trust an atheist than a so called christen. Run the other way if they say "you can trust me" because they believe in Jesus or the lord. Generally I find a non believer far nicer and kinder that these God loving types.

There's also the Christian God's own admission that he's responsible for evil:
Isaiah 45:7: "I form the light, and create darkness: I make peace, and create evil: I the Lord do all these things."

I'd retort that there is nothing moral about renouncing logic in favor of superstition.

I also point out some of the life instructions in the Bible, which is the basis of their belief. Should fathers really their daughters to their rapists? Should you really kill someone for working on Sunday or for not sharing your belief? (://www.youtube.com/watch?v=vkXOwBIRX7Y)
Hitchens pointed out that, left to their own devices, good people will do good things and bad people will do bad things regardless of but, to get a good person to do evil requires . I'd broaden that to: to get a good person to do evil requires a delusional belief system (or substance abuse), of which is certainly one, but I'd include some political beliefs also. Were all the Germans who fought in WWII evil, even though they fought under the leadership of arguably the most evil person ever? I doubt it.

Sam Harris elucidates the fallacy of using the Bible as a moral guide here:://www.youtube.com/watch?v=vSdGr4K4qLg&t=79s
He also gave an excellent TED talk entitled "Science can answer moral questions." (://www.ted.com/talks/sam_harris_science_can_show_what_s_right)

The major religions do not even have a common ground about morality.

Your morality comes from the environs and social construct into which you were born and raised. The morality of your society where you live has arrived to its point from many years of mostly secular debate and laws. You will look at another society and think that it has immoral viewpoints and they in turn will look at yours and question your morality. Morality does not come from an absolute authority i.e. < insert random deity of choice here >

Within your own social structure you will find people you consider to be without morals due to your own perceived thoughts on morality but live there lives at a high moral standard and you will also find people that seem to be utterly devoid of morality compared to the society in which you live.

It's all about where and when you were brought up. If it was deity based then it would be designed into us or if religion based then you would have a hard time explaining why so many priests and religious authority figures of all nominations are so corrupt and find children so sexually alluring

There is no morality in the Torah nor its offspring The Babble and The Qu'Ran yet for some reason people of all three faiths seem to think they have the higher moral ground than atheists.

E.g. In the Babble Lots daughters wind up in a cave with their dad having just escaped destruction in Sodom where they were offered, by their dad, to the towns folk to be raped instead of raping some angels that were staying with them. After losing their mother they wonder how their father was going to have a son to carry on his name so they both get him drunk and have sex with him and both instantly become pregnant. Praise Da Lord!

Just one of the Babbles many fine tales of morality.

Professor Stick does it well

check out the rest of his channel

If there is a God he will have to beg my forgiveness.

People say daft and stupid things when they are confronted with a higher intellect, something which you obviously display given that you are an atheist.
I have exactly the same problem although I do have many Christian friends and some of them are quite intellectual as well although not quite as intellectual as an atheist.

Morality should be based upon compassion and evidence only.

Why would you strive to put someone else down?

Why settle for just one quote?
[google.com]

I say something general like "different strokes for different folks." I'm sure others have more exciting comebacks.

If you didn't have morals you would literally be taking a piss where ever you liked or walk around dressed as you like or undressed. you would just take things from anywhere without paying. you would have been locked up years ago. ironically there are a lot of prisoners who turn to God or as I like to say, kiss ass to get out early. you won't faze these hardcore believers at all.they would have to except that their whole life is a lie and maybe/probably a lot of there friends and relatives.

Christians have a moral code, they just don't live by them. I have found Atheists to be much more upstanding in the morality department, maybe it's because we have to walk our walk instead of just talking the hypocritical religious talk. I am always amazed when the religious people try to claim the moral high ground when their history shows them to be completely incapable of acting in a moral manner. If it takes the constant threat of being cast into hell fire for eternity get a person to act in a moral manner then they aren't being moral, they are just being obedient.

A moral person listens to his conscience, i.e. to a great degree, to the chinese Golden Rule (later filched by Jesus) and does not have to dig his morality out of a book!