There are many knowledgeable Muslims, who are doubtful about Islam. But their journey of doubting Islam ends with this question: "What was there before the Big Bang?" And when they don't find the answer to this question, then they keep on sticking to Islam and taking Allah as a true god. But this is wrong, and it happens only while they have this impression that in order to leave Islam, they have to first deny the existence of any god (i.e. they have to become atheists). 

But this is only a misunderstanding. There is absolutely no condition of denying the existence of any god/gods in order to leave Islam.

Science has yet not found from where did the 'matter' came. Thus, till this question is fully answered by science, every atheist will keep on having a part from 0.01% to 49% of 'agnosticism' in him (i.e. there may exist any god/gods). Please watch the following video of Dr. Richard Dawkins to understand it clearly.  

Thus, it is not necessary to find the answer first from where did the matter come in order to leave religion. But we only need to deny any religion which claims that it's "religious god" convey the message to humans through angels. 

Please note that the discussion of presence or absence of any god is an "internal" discussion among Atheists and Agnostics and Deists. While the discussion with religious people is only about their "religious god".

Moreover, do you know that Muslim Preachers are teaching these TACTICS to Muslims in private gatherings that they should engage us only in the discussion if god exists or not, so that we don't get a chance to do a Direct Criticism upon Allah and Islam. 

It is a reality that Allah/Quran/Islam has no chance to face the direct criticism of Humanity and modern science. Either it is the Islamic Slavery, or oppression against women, or rape of minor girls in name of marriage, or scientific mistakes in the Quran and Hadith, then the religious god (i.e. Allah) is immediately EXPOSED. 

Therefore, please don't get into the trap of Muslim preachers, and don't indulge in an unnecessary debate of existence of god, but target and criticize Allah and Islam directly in the first step and prove them wrong.