What is the sin incurred by the father if his children's upbringing was not sound, or if it was not according to Islam, after their foreign mother took them and traveled with them to her country following a dispute between them, and what is his duty towards his children in this situation?

A father should not leave his children to be raised by a foreign, non-Muslim woman, fearing that they might be brought up in a religion other than Islam, especially since custody is a guardianship. The majority of scholars stipulate that the custodian of a Muslim child must be Muslim, based on the Almighty's saying: "And never will Allah grant the disbelievers a way over the believers." The father is responsible for raising his children in Islam, and he must fear Allah concerning them.