The X-shaped cross is a St. Andrew’s Cross, and originally WAS a religious symbol in Scotland. as St. Andrew is the Patron Saint of Scotland. But the St. Andrew’s Cross is probably not religious in Jamaica or Brazil or the Confederacy except passed down from some other flag.
True Christian symbols on a flag, if they existed, would be things like a Lamb and Crown, Rod and Staff, Fish symbol (ICHTUS) or—well, there’s the Christian flag itself but it by definition is not a national flag…