Book “Faidat jamiat min kitab an-Najmu Saqib fi Ahwal imam al-Qaib” written by shia shaykh Ayatullah Husayn at-Tabarsi an-Noore (p 6):

Translation:

From Imam as-Sadiq (alaihi salam): No one except believer would born in the night in which would be born imam al-Qaim. And if he would born in the land of Shirk, Allah would switch him to the belief.

So basically this report mean that each and everyone who was born in the night when was born shia Mahdi was believer, or if he was born in France (for example) Allah brought him to Faith!