Paul suggests in Romans 1 that the fundamental obligation we owe God is thanksgiving.
For he says the fundamental sin is to refuse to glorify Him or give thanks (v. 21). The implication is that either giving thanks is part of what it means to glorify God or the primary way in which we do so.
In either case, it is at the heart of our fundamental duty as creatures–to give thanks to the Creator.
But thanks for what?
Paul says just before this that the creation (think here of the universe and the human person, as Paul states in 1.20 and 2.15) shows God’s eternal power and divinity.
Let me suggest that this means we are to look up at the starry heavens and out at the beautiful world, and within to our moral nature, and give thanks for God’s beauty and power.
In other words, we owe thanks for the mystery of existence–simply for the privilege of being. After all, God did not have to make me. Or to place me in this beautiful world I have been given to enjoy.
Or to enjoy all the innumerable little blessings of day to day life–such as family and health and love–and the immeasurable gift of knowing the Triune God.
Giving thanks brings pleasure. The act of thanksgiving sanctifies each gift when I hold it up in thanksgiving.
So St. Paul would commend (is commending?) the American republic for devoting a holiday to what we were created for. To encourage us to do our most fundamental duty as human creatures–to give thanks to God for all that we are and have.