In that case we are all Muslims. According to Islam, if you have once been a Muslim, then you can never not become a Muslim.
No... That's more of a Christian thing, "Once saved, always saved". However in Islam, you've got to keep your guard, you can still die a kaafir and get Jahannam.
Now that I think about it, I remember a hadith that says, "even if once in your life you had a mustard seeds worth of faith, you will enter Jannah, eventually." So I wonder if murtads stay in the lowest level of hell for eternity, or we have a shot at getting parole?
According to Islamic theolgy, aren't we all born Muslims ? ... that makes us all Ex-Muslims
...... from the great satan
That's a misnomer. All of mankind is born upon the 'fitrah', the natural inclination towards monotheism. Not muslim.