I married my Muslim husband 9 years ago in my country while I was Christian in papers but atheist in reality and I was not a chaste person. Now I became Muslim from 6 years, even I am a devout Muslim now dressing Niqab and we have 3 children. Is our marriage valid?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

We congratulate you on your acceptance of Islam and determination to adhere to the Islamic rulings in your day-to-day life. May Allaah pour His blessing upon you, grant you happiness in both lives, and make you steadfast on His religion. 

Islam prohibits marrying a non-Muslim woman except if she is a chaste Christian or Jew. Allaah says (which means): {And do not marry polytheistic women until they believe .....}[02:221].

Therefore, if you were atheist at the time of marriage as you have acknowledged in your question, then your marriage is invalid. You have to separate immediately from your husband and stop sleeping with him since he is not your husband according to Islam.

Now after completing the waiting period you must renew your marriage contract fulfilling all the conditions of a valid marriage as we have mentioned in our Fatwa: 83629.

However, the children are traced back to your husband as long as he married you on the assumption that the marriage with you was correct.

Allaah knows best.