Is it permissible to combine Islam and Christianity in this era under the pretext of the descent of Jesus (عليه السلام) at the End of Times?

The combination of Islam and Christianity under the name "unity of religions" is clear disbelief (kufr), because it mixes monotheism with polytheism. A Muslim's Islam is not valid except by affirming monotheism and disbelieving in taghut (false deities/idols). As for interfaith dialogue and arguing in the best manner, that is permissible. The descent of Jesus (peace be upon him) is a truth, and he will rule by Islam, not by Christianity or Judaism. He will break the cross and kill the pig, thereby invalidating the Christian religion.