Insha Allah this post will help you in clarifying your misconception
https://yahyasnow.wordpress.com/2009/08/30/28/
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[TD]Who was the First Muslim in the Quran? No Contradiction/Error! – Yahya Snow's Blog
a refutation of the attack on the Quran concerning who was the first Muslim of Beleiver...[/TD]
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There are two things that need to be highlighted here:
1 Islaam was always there. Every prophet was a Muslim so Islaam didn't start roughly 1400 years ago
2 one is not a muslim since his birth. Our souls already existed long before Adam as walked the Earth. There is this "Qalu Balaa" event that took place where Allah gathered every souls and asked us:"Am I not your Lord?" and we responded with "No you are!". We are considered muslims since that event....long before Adam as was send down.