If a man divorces his wife and realizes so (by asking scholars) after six months, thinking all the while that no divorce took place, does he have to renew the marriage contract?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

If this husband initiated divorce, then divorce has taken place, and the fact that throughout this period he believed that divorce did not occur is of no effect. The waiting period is counted from the time when divorce is issued. If the waiting period has expired, he is not permitted to take her back except with a new contract. This is, of course, if this divorce is not the third divorce. If it is the third divorce, she is forbidden for him until she marries another husband, as Allah Says (what means): {And if he has divorced her [for the third time], then she is not lawful to him afterward until [after] she marries a husband other than him.} [Quran 2:230]

But if this man has had intercourse with his wife before the expiry of her waiting period, then in our view according to the most preponderant opinion taking her back is effective.

The Musannaf of Ibn Abu Shaybah reads: “Al-Hassan al-Basri was asked about a man who said to his wife: “If you enter the house of so and so, you are divorced one divorce.”; and she entered without his knowledge; so Al-Hassan said: “If he had sexual intercourse with her during her waiting period, then his having sexual intercourse with her is considered as taking her back.’ [End of quote]

Finally, it should be noted that the Muslim should take the initiative to ask the scholars with regard to the rulings that he needs to know before acting, as Allah Says (what means): {So ask the people of the message if you do not know.} [Quran 16:43]

Allah knows best.