Depends. What I say is usually a criticism of whichever empty assertions or logical fallacy they have been indoctrinated in but probably never really challenged on.

It's all lies. I don't know of a single theist who doesn't lie about something when promoting his/her religion, and that lie is all you need to dismantle to break the argument.

It isn't always spoken out loud.
"Arguments" like 'you can't prove there isn't a god' may be truthful, but the insinuation is that 1) religion can prove gods exist 2) non-believers claim they can prove gods do not exist 3) there are no other gods than whichever deity they worship 4) since we don't know anything, we may as well believe in their deity.
All of which is false.

"I feel god's presence" may also not be an outright lie but it dismisses the professed divine feelings of each and every single individual throughout history who follows a different religion, and files non-believers whom do no feel god as broken or flawed.
Feelings = truth, is a lie.

Explain to me why Zeus doesn't exist, then apply that to your god.

I think Ricky Gervais said it most succinctly.

This has some of the basic ideas that blast christianity in a serious way, ideas that I often use against them:

Some of the Reasons Why I Am Not A Christian . . . .
Many people, especially those who profess Christianity, believe that theirs is an omnipotent God, who created the universe, who is "love". But, if you could create a universe, and you were indeed the embodiment of love, would you create a universe were beings of that world would prey upon other beings.......a world where it is necessary for beings to consume other life in order to live? I think not.

Suppose that you go to your doctor because you are sick, and after the usual check-in, you are brought to the examination room. When your doctor comes in, there may be the short bit of small-talk, then he asks you your symptoms. You tell him, and then, to your utter astonishment, he says "Unless you acknowledge to me, that I am a doctor, and address me as Dr. Smith, I will not treat you." I think that the average person, with any common sense, would not wait around to be treated by that so-called doctor! This analogy can be applied to Jesus. Why does he insist that others acknowledge his title? Would someone with a normal bearing in life, having a title, (as would a true doctor) feel a need to have their title acknowledged?

When there is a great social stigma attached to a particular thing, it is not uncommon for people to do everything they can to hide this stigma from society. In historical times, fornication was considered a very serious sin. What better way to attempt to convince others that you were indeed of legitimate birth, lacking any other proof, than to claim to have descended from God himself! So much for immaculate conception.
The idea of "original sin" is preposterous, from a humanistic perspective, and from a realistic perspective. It is simply a dinosaur that was carried from the past, when the disgrace of an individual was considered the disgrace of every generation afterward. The average person does not hold things against children who error for the rest of their lives or for eternity, for that matter. Besides, one does not learn, without making errors.
There are plenty of people who profit by keeping others away from enlightenment. The dissemination of lies is motivated mostly by greed, but also the want of power. It is a parasitical relationship.
Some people are just lazy..............they don't want to even take the time to actively try to be of any benefit to humanity, so they give their money to the priest, whom they assume (a lot of times falsely) will do good for them, or at the very least, absolve them............. These priests are suppose to be wise men and are suppose to give guidance to the general public . . but they are also suppose to live a life of semi-seclusion and remain a virgin all their lives. If I want to learn how to fire a cannon though, I'll go to someone who has done it, thank you.

Who could imagine a parent so pernicious as to condemn their children to suffer eternally? Yet, when the idea of eternal punishment is supported by biblical text, it seems to be ok to people....... If you love someone, placing conditions on that is counter-productive. As Nietzsche would say, "Whatever is done from love occurs beyond good and evil".

And, last but by far not least, religion is a prejudicial mental state of separating yourself from others . . . . from the earliest times, religion has encouraged man to believe that he is above the animals in this world, and, not only the animals, but also other humans who do not embrace his "god". This concept of separation from the class, "animal", leads man to believe that he is justified in anything that he does to any "lower" sentient being, up to and including other humans who happen to not buy into his bull shit story concerning "god". This is, in fact, one of the main reasons why governments, almost always, without exception, support some form of religious belief, because it can be used to justify mass murder of those who are deemed as non-believers. History is full of this stuff, yet they claim it is "LOVE", or the "Will of God". Hogwash, it is evil, and it is evil to the core!

i have no need to break down theists. no one who knows me tells me i am foolish for not believing, and strangers who try to do so don't get a lot out of me. however, it can be done with two words: "talking snake."

g

Set a good example of being a human being, is the best, if hardest thing you can do, then maybe one day they will ask the questions themselves.

Help them deconstruct why and how they believe what they do. With compassion and genuine understanding. Cuz, well, everyone has a story, right?