Atheists know there is a God
Theists claim that the existence of God is so axiomatic that nobody can deny God. Atheists do not really exist, since they are simply suppressing their knowledge of God. Rather than actually providing proof or valid argument for their God claims, they resort to claiming that the evidence is all around us. Others may claim that we already know God because He created our souls and gave us life, or that God is a part of each of us. Christians, for instance, often cite Romans 1:20 "The invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; that they are without excuse." Basically, the claim is that the evidence for God's existence is so clear that to deny it or fail to accept it is inexcusable. Based on this assumption, theists may try to paint atheists as hypocrites by claiming that all atheists merely pretend not to believe in God.
God does not obviously exist
It is not axiomatic, obvious, or self-evident that there is a God. Most or all atheists have no experience of "knowing God" or any feeling of suppressing such knowledge. Rather, it seems the theist who makes this argument is blinded by his or her own faith-based beliefs. He or she may be under the delusion that because such beliefs of self-evidence are so cherished by the individual, they must also be universally experienced in the minds of every human on Earth. Unfortunately for the theist and the atheist alike, there is no way to know what any given human actually thinks or believes, so this argument depends on a great deal of wishful thinking.
The claim that "atheists know there is a God" is probably both non-verfiable (at least given the current state of medical science) and non-falsifiable. Indeed, any failed attempt to prove an atheist doesn't believe in gods (polygraph, FMRI, etc.) could be construed as either a failure of the equipment to detect deceit, or as deception by Satan, or any other ad hoc/post hoc explanation. The efficacy of polygraph tests has been questioned, and evidence against their reliability as a tool for the detection of deceit has led to their inadmissibility in many courtrooms. This aside, the theist who is claiming the atheist knows there is a God likely has no reliable, accessible means by which to substantiate their claim.
If a God would be expected to provide evidence of their existence or reasons to believe in him, the lack of evidence is itself an argument against the existence of God.
An atheist could simply turn this accusation around and claim, "Theists know there is no God. In fact, everyone is born with the knowledge that we are just a natural product of the universe!" A responsible atheist should make no such assertion, as both the "God exists" and "God does not exist" forms are equally indefensible. Of course, this assertion's main purpose is not "proving" the lack of existence of gods, even less giving factual data about what people really think, but simply serving as a reverse-argument that is, logically speaking, just as valid and sound as the apologist's argument. If it is easy to spot the ludicrousness in the claim "You know that universal invisible sky pixies exist, but you just pretend not to believe," it should be easy for a theist to recognize that arguing about the inner thoughts of an atheist likewise is obnoxious.
As for appeals to Bible verses like Romans 1:20 (cited above), such appeals represent a fallacious argument called the argument from scripture. Besides, relying on religious scripture does not tell us which specific scripture to use; using Bible verses is just as valid to Christians as using verses from the Koran is to the Muslim. For instance, a Christian can deny Allah all he or she wishes, but according to the Koran, they are without excuse. The Primordial Covenant in Islam states that Allah created our souls and we all have testified that he is our God before we were even born (Surah 7:172-174 : "When thy Lord drew forth from the Children of Adam -from their loins- their descendants, and made them testify concerning themselves, (saying): "Am I not your Lord (Who cherishes and sustains you)?" They said: "Yea! We do testify!: (This), lest ye should say on the Day of Judgment: "Of this we were never mindful;" or lest ye should say: Our fathers before us took false gods.) It is common for religions to include a system that links all human beings as being created by a specific creator and claims humans are without excuse for disbelieving. There is no clear way do determine which gods are real and which are imaginary. Thus, the presentation of any single one of these systems is not particularly impressive to the atheist.