Some words on submission

So; the committed Muslim & the one in doubt BOTH know about the "issues":

The existence of hellfire, existence of evil or offensive warfare to conquer disbelieving lands (jihād at-talab), concubinage, the marriage of the Messenger ﷺ with Sayyida A'isha, the hudūd etc.

So what makes the difference?

It truly comes down to submission. Submission is what separates both groups. Most of the heresies we find in modern man are rooted in (if I was to summarize) man's inability to realise that he is the slave and Allāh is the Lord.

One must know, numerous apostasy cases include huffāz of the Qur'ān, or those who were once upon a time giving da'wah themselves, and those who do actually know about the religion.

So know, that one could learn all the rebuttals against the arguments of Islam thrown at him from an intellectual standpoint, but still have uncertainty in his Islam, simply because his heart never completely submitted, for imān isn't just an intellectual exercise for the mind, rather it's a spiritual conviction in the heart (that Allāh gives His slave) tied with the intellectual certitude.

Imām Abū Ja'far Al Tahāwī beautifully summarised:

"For none is safe in his religion except those who surrender themselves to Allāh Almighty and to His Messenger (ﷺ), and relinquishes the knowledge of what is ambiguous to the One who knows it.'' The Imām also says: "A man's footing in Islām is not firm unless it is based on SUBMISSION and surrender."

The one in doubt thinks "how can Allāh possibly legislate this?! Whereas he committed Muslim is content in: whatever Allāh wills and whatever He revealed.

One can argue, only if these people who had left the religion grounded themselves in foundational 'Aqīda (creed) with a firm grounding, I say:

This is a fair point, yet first of all we must know that Allāh guides whom He wills and secondly one should realise that even if people learn the foundational creed for a 1000 years from the greatest theologian to have ever lived, BUT if this person never internalised what he learned in his heart by complete & total submission to Allāh, His Will, His Decree, His rulings & His Divine Wisdom, he would still waiver!

This is why it is not a matter of just knowing, but a matter of submitting like the story of Sayyidunā Ibrahīm, Allāh raise his ranks, was commanded to slaughter his own!

So what truly separates the believer and people of doubt and those who went astray is:

Submission - to know that we are broken slaves of Allāh who have limited knowledge and affirming Allāh is our Lord, the King of Kings!

And Allāh knows best.