The tradition of red doors dates back centuries. There are several meanings associated with red doors.
Red symbolizes the sacrificial blood of Christ that saves those who come to him. Red is also a reminder of the Passover and a sign of the Holy Spirit. Church doors began to be painted red as a way of remembering the ultimate sacrifice that others had made for their faith.
The red doors of churches traditionally indicated a place of sanctuary, refuge, and safety. Those in need would not be captured or harmed inside the holy walls of the church, which offered physical and spiritual protection.
Some believe that the doors of the Wittenberg Cathedral, where Martin Luther posted his 95 theses, were red. Red doors on Protestant churches indicate their roots in the Reformation.
Whatever the meaning, many churches today are recognized by their distinctive red doors.
Sauce: https://www.umc.org/what-we-believe/why-do-so-many-churches-have-red-doors