The Great Work comes in two parts. Discovering who you are not and discovering who you are. Technically you already know who you are but it is hidden by who you are not.
Years ago, I discovered that the truth is revealed by the lie. The person that shouts, "There is no God!" knows deep down that he fervently believes in God. Then why does he say this? It is only a partial lie. His conscious self mostly believes what he is saying. The truth isn't found by convincing this person that God does exist. The truth is found in the motivation for the emotion that inspired the statement.
He may be saying these things because he feels unworthy of God (fear), that he will lose you to your relationship with God (insecurity) or that he had a very bad experience on his quest to God (pain). Once his emotional base is healed, he rediscovers his connection deity and who he is.
Some lies are apocryphal stories people tell about themselves. These are reveal a person's own mythology and are used to share attitudes and beliefs about herself with others. These result in less self-damage than the type of lies mentioned above. They do indicate difficulty in direct communication and self-awareness. The latter because she allows herself to believe stories she tells that never happened.
The point of this isn't about the other guy, it is about you. You know when you are lying. Your job is to figure out why. The difficulty there is that too much of you believes you are telling the truth and only a small part is aware of the lie. So how do you ferret it out?
Clue number one is anger. If someone tells you that your god does not exist and you get angry, chances are that your identity has been confused with your deity. The lie here may be I have no spirit only belief. This runs so deep that if asked you could honestly say you don't believe it but it is revealed because your ego was so easily bruised by someone else not believing your belief. What you think of as your strength (your belief) is really a weakness. Belief is weak, easily doubted, and as useful as a paper sword. When you know, rather than believe, challenges to such things reveal more about the challenger than yourself, your demeanor is calm and your heart open. Anger doesn't enter into it save someone being particularly and persistently obnoxious. You cannot allow that caveat to be an excuse for not deeply examining your anger. If you are getting angry at a two-line Facebook post, a reasonable counter-point or simply a different perspective, it is you not the other person with a problem.
Clue number two is the need to qualify. Truths can be shared with simple direct statements. The universe unfolds with perfect beauty. Lies need qualifiers. The universe unfolds in perfect beauty but you shouldn't do that because it is wrong. The second part reveals the first part as belief, not knowledge. The lie here is that one knows when the statement is just someone else's truth is being repeated.
Truth is expressed in these simple statements but is often taught via the complexities of perceptions, the mind and/or systems such as the Kabbalah that are so complex that they busy out the robot. Also, the expression of the truth does not necessarily mean it is fully integrated. One can know but not know. Integration is revealed by one's state of being rather than a spoken truth.*
Clue number three is pejorative language. If you have to declare someone with a different view, set of traits or experiences is dumb, crazy, defective, or make marginalizing statements about that viewpoint or person, you are revealing a truth; My ego is too fragile to accept that I may be in error. This is insecurity. The truth could also be something like, "Only my experience is valid." This is a problem of ego inflation.
The healthy ego is in the Tao. One cannot be in the Tao while being angry, insecure or false. All of those negative expressions are the embodiment of who you are not.
The good news is that simply allowing yourself to see this stuff begins the healing process. Chances are you will stop yourself from saying it by telling yourself the deeper truth that you can't quite own. Oh, they just have a different perspective, that is okay. In time, your feelings will be of reduced intensity but you will still need to repeat the truth to stay composed. Eventually, you will simply not react that way at all. That in turn opens the door to deeper understanding of yourself because everything up to this point is about the other guy and the deeper lessons are never so external.
*paragraph added shortly after initial posting