Mary, in my understanding Muslims believe that *every* child is born pure and submitting to one God,wihout any Shareek i.e partners .
That is to say every child is BORN Muslim and it is only later that the child's parents/soceity RAISE him to be a Christian,Jew or any other Faith.

Thus many Muslims say when some non-muslim becomes Muslim that so and so has reverted to Islam .That is to say they are back to the religion they were born on .

Massalama.