It is what it says.

"Devotion is neither private or public prayer, though private and public prayers are part of devotion. Devotion signifies a life given or devoted to God." This is the profound but simple principle that William law 1686-1761 expounds in A Serious Call to a Devout and Holy Life. He is basically speaking against "Sunday Christians" who are pious on Sunday and live a life that bears no resemblance to what they profess every other day of the week. They just do their own thing!

He is basically speaking against those have no outward expression of devotion to God beyond their Sunday attendance or their prayer life. Scratch the surface and you will find not much there. he closely aligns with  the Apostle James in that faith will translate to works and an outward expression of what we pray study and read in private and at church. "Faith without works is Dead"

There is no more to be said it profound yet simple. Do you walk the talk?