Martin Luther once said this:
The believing man has the Holy Spirit; and where the Holy Spirit dwells, He will not let a man to be idle, but stirs him up to all kinds of godliness and good works, and of true religion, to the love of God, to the patient suffering of afflictions, to prayer, to thanksgiving, and the exercise of love towards all men. http://www.azquotes.com

This is what St. Paul calls our spiritual “obligation”: to live godly lives as the Holy Spirit “stirs us up”. This is your obligation to the Lord; it is what is expected of Christians; it is your duty and responsibility in faith. Theologically, it your response to God’s mercy in the Lord Jesus Christ.