Eisenhower added this to suck up to the Jesus freaks.
He also strted the silly ritual of "prayer breakfasts".
Believers, agnostics and atheists can all be loyal to the US Constitution and the flag. No belief in God is required to be a patriot.
Claiming that the flag and/or the country are "under God" does not cause God to exist.
All countries are "under God" if God exists, and if he does not exist, none are.
Adding "under God" to the Pledge is just plain flat-plant silly from any logical point of view.