I am confused about how the "line of sight" works with other civs citys that have the same religon as yours.

can anyone help?

Thanks in advance

When you control a religion's holy city, and have adopted that faith as your state religion, the fog of war is lifted around any enemy cities where that religion is present.

Note that, since your state cannot have 2 official religions, you only get line of sight from one holy city at a time. And, you lose it if you adopt freedom of religion.