"The commencement of godliness is the love of God."

John Calvin on the greatest commandment (Deut 6:5; Matt 22:37 and parallels):

[N]o man will actually obey God but he who loves Him. But as the wicked and sinful inclinations of the flesh draw us aside from what is right, Moses shows that our life will not be regulated aright till the love of God fill all our senses. Let us therefore learn, that the commencement of godliness is the love of God, because God disdains the forced services of men, and chooses to be worshiped freely and willingly; and let us also learn, that under the love of God is included the reverence due to him.