1954 I believe -the same year Congress provided the prayer
room with it's decidedly Christian images displayed in the
stained glass window. whatever the motivation it(under God)
does quite nicely tie the Pledge to the National Anthem,and to the national motto-and to that beautiful
flag folding ceremony used at some of our embassies,and at
the Air Force Academy ( or it was prior to the recent false
charges brought against Gen.Weida.)
That is 51 years ago.
It's purpose was secular and not religious.
And no national religion has been established that I can see. At least I haven't been required to attend one.