Should I stop seeking forgiveness for my father, who used to pray but did not believe in resurrection after death, rather he saw prayer as lessons that benefit us and that humans decay and perish after death, fearing that I might be among those who show affection to those who defy Allah and His Messenger?

Indeed, belief in the Resurrection and the Last Day is a great pillar of faith, and its denial is considered disbelief by the consensus of Muslims. However, uttering or performing an act of disbelief does not necessarily mean that the person is instantly judged as a disbeliever, as the ruling of disbelief on a specific individual is contingent upon the fulfillment of conditions and the absence of impediments. As long as your father demonstrates adherence to Islam, the default is that he remains a Muslim, and the rulings of Islam apply to him. Therefore, there is no harm in seeking forgiveness and praying for him.