Religion plays a major role in our identity. The first documentation of the word Somali was during the Adal-Abysinian war when the king of the habash was writing a letter to someone. That was when Islam had a huge grip on Somalida and most of us were Muslim. Our ethnogenesis as a Somali people happened because of Islam and that's because the only time our people ever united was when we were fighting against gaalo. The use of the word Somali increased during the colonial era when Somalis needed to unite to successfully fend off these invaders that wanted to make us subservient to them by conquering us and changing our religion.
When reading about wars that were fought by Somalis, most would consider them wars that were between native Somalis and foreigners, but the old Somalis didn't see it that way. They considered it a war between religions. That's why they addressed their enemies as Gaal Madow(Oromo, Xabash) and Gaal Cad(Portuguese, French, British, Italians) because what all these enemies all had in common was their intention of sabotaging and changing our way of life and a major facet of our identity.....our religion. One of the roots/base/foundations of the concept of Somalinimo is Islam, because Islam is the one that helped in unifying our people.
Now tell me, how can an apostate call himself Somali when he has abandoned such a major facet of our identity? A facet of our identity that was one of the main causes if not the main cause of our ethnogenesis as a people. A Somali that has abandoned Islam has also abandoned Somalinimo. Simple as that.