In the real old days, the king was called the defender of the faith. I guess I'm a downright reactionary, but I believe that the Bible teaches that the government is supposed to support and defend the worship of the God of the Bible. God raised up nations and tears them down. As such, they owe Him their worship and obedience.
This hooey that we all should be free to worship the God we choose is just that -- hooey. I am never free to do that which is wrong. Therefore, I could never be free to something as sinful as denying the Creator. The only way we even know or understand true freedom is in the first place is through the revelation of God.
The phrase under God suggests that this nation is under his rule and dominion. It also indicates that it is under his blessing and protection. If we as a nation deny that our nation is under God, who do we then rely upon for protection and blessing? Do we trust in our own strength and ability? The very thought is enough to make this Texan literally shudder in his boots.