As'salamu Alaikum dear brother Syed,
Your argument would not work on agnostics. It would certainly work on atheists, but the reason it wouldn't work on agnostics is because they believe in GOD Almighty but reject all world religions as false man-made lies. So by us Muslims showing them the Miracles of the Glorious Quran, then we can insha'Allah win many of them to Islam.
One who rejects bowing to a cow, for example, is not one who rejects GOD Almighty. I hope this helps insha'Allah.
Take care,
Osama Abdallah