Atheists know there is a God
This argument is made against atheists. Theists claim that the existence of God is so axiomatic, that nobody can deny God. Often times rather than actually providing proof or valid argument for their God, they resort to claiming that the evidence is all around us. Or they claim that God somehow is either a part of everyone or since God created our souls/life we, therefore, already know him. Christians in particular 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 evidence is so clear that no one can deny it or be excused from accepting it. Based on that assumption, theists claim that all atheists just "pretend" not to believe in God and paint atheists as hypocrites.
The claim is both non-verfiable and non-falsifiable; any attempt to prove you don't believe in gods (polygraph, FMRI etc) will be construed as "you beat the test", "Satan is deceiving you", or any other ad hoc/post hoc explanation.
For the most part, it is not axiomatic, obvious, or self-evident that there is a God (feel free to further explore this site to see why). Rather, the theist who makes this argument is completely blinded by their own faith in their own personal beliefs that they have deluded themselves into thinking that their beliefs are universally shared and accepted internally by every human on earth. The reason why this is delusional is because there is no way to know what every human thinks or believes, and thus this argument depends on a great deal of wishful thinking.
It is also a terrible attempt to claim that the person knows the atheist better than the atheist. Just as a person could easily spot the ludicrousness in the statement that they know that universal invisible sky pixies exist (and they just pretend that they don't believe), a theist should know better than to obnoxiously assert that atheists pretend not to believe in their particular god.
Most of the time, the theist who uses this argument does not have a leg to stand on. Therefore, the theist retreats to wishful thinking.
As for Bible verses like Romans 1:20, this is an fallacious argument called Argument from scripture. However, it all boils down to circular reasoning, and frankly using Bible verses are just as valid as claiming the statements from any other holy text is just as equally valid. For instance, a Christian can deny Allah all they wish, but according to the Koran, they are without excuse. The Primordial Covenant in Islam states Allah created our souls and we all have testified that he is our God before we were even born (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 humans are without excuse, such as "thetans" in Scientology. The thing is, it is all faith-based and impossible to empirically verify with any evidence.