Why do ex Muslims think that they are experts on Islam?

(1) No Advanced Qualifications are Required to Join or Leave Islam:

To become Muslim or to leave Islam, you don't need a PhD or any advanced degrees. Having a basic understanding of the reasons for or against Islam is enough to make a personal decision.

Similarly, when non-Muslims convert to Islam, they often do so without deep academic study. Their knowledge of Islam is far inferior compared to that of ex-Muslims. Yet, it's acceptable for Muslim preachers when these new converts embrace Islam.

However, when ex-Muslims, who often possess a deeper understanding of Islam, choose to leave the religion, there's suddenly a demand for high-level expertise as justification. This shift in standards are nothing else than Double Standard.

It raises the question: Why is basic knowledge acceptable for joining Islam, but leaving it requires expert-level credentials?

(2) We don't need to be experts on the entirety of Islam, but a single inconsistency or injustice attributed to a supposedly perfect deity is enough to raise questions about the faith

According to Islamic claim, Allah is 100% Perfect and can never commit a mistake. 

Thus, if it is proven that there exist even a SINGLE flaw or mistake by Allah, then the rest 99.99% of Islam will be automatically proven wrong. 

For example: 

• Quranic story about the CREATION goes completely against the scientific Theory of Evolution. The Quran has no chance against science. It is not needed to waste time any more, and this single Quranic mistake is enough to leave the rest 99.99% of Islam too. And there are so many Scientific Blunders present in the Quran. 
• Similarly, the issue of "Slavery and Sex Slavery" is alone enough to leave Islam, while it become proven after it that Allah is UNJUST, and an unjust entity can never be a GOD. 
• Even today, in the era of mass communication, hardly 0.000001% of people convert to Islam. This means, the rest of 99.999999% of innocent people, who by Allah's DESIGN take birth in non-Muslim families, will become fuel of eternal hellfire, despite their good deeds for humanity. This again proves that Islamic god (i.e. Allah) is UNJUST. And an unjust entity can never be a GOD. 

(3) It's Not Just About Knowledge; It's About Doing Justice:

In Islamic teaching, Satan is considered highly knowledgeable—so knowledgeable that no Islamic scholar can surpass him. However, despite his vast knowledge, Satan is believed to have gone astray because he failed to do justice with the facts.

Ex-Muslims don't necessarily claim to be more knowledgeable about Islam than Islamic scholars. What they do claim, however, is that:

• Islamic scholars are deeply indoctrinated by religious teachings.
• This indoctrination leads to blindness. The evidence that Islam is flawed is right in front of them, but rather than using their knowledge to seek truth and do justice, they focus all their energy on defending falsehood.

(4) The Decision Isn't Based on Knowledge, But on Arguments and Proofs:

In a court of law, the outcome of a dispute isn't determined by which lawyer has more knowledge. Instead, it's based on the "Arguments and Proofs" presented by the lawyers.

Even the Quran does not suggest that knowledge alone is the standard for determining what is right or wrong. Instead, it challenges skeptics with the words: "Bring your proof if you are truthful." (Quran 2:11)

As ex-Muslims, our decision to leave Islam was based on "Arguments and Proof." If Muslims wish to bring us back to Islam, they must convince us with compelling arguments and solid proofs. Simply criticizing us for not being as well-versed in Islamic studies as their scholars won't achieve that.

(5) Ex-Muslims Have Greater Insight into Islam's Dark Side than Average Muslims:

Although we don't claim having a PhD degree about Islam, but we still believe that, on average, ex-Muslims have a deeper understanding of Islam compared to typical Muslims, particularly when it comes to recognizing the darker aspects of the religion.

Islamic scholars have created an isolated environment where only one-sided, favourable information about Islam is shared. They highlight the positive aspects while completely suppressing any criticism that could shed light on the darker sides of Islam.

Any criticism of Islam is immediately branded as blasphemy, leading to harsh consequences such as imprisonment or even killing those who dare to criticize Islam. This Muslim behaviour is a great injustice. Muslim communities must be ashamed of themselves for committing these crimes against humanity. Not even the polytheist pagans did so much ZULM upon Muhammad. Muslim communities are far ahead in committing this ZULM against non-Muslims. 

Furthermore, we also do believe that ex-Muslims, on average, have a higher IQ than their Muslim counterparts. Most ex-Muslims are atheists, and studies suggest that atheists generally exhibit higher IQ levels compared to religious groups. (Source)