When we are called to follow Christ, we are summoned to an exclusive attachment to his person.
The grace of his call bursts all the bonds of legalism. It is a gracious call, a gracious commandment. It transcends the difference between the law and the gospel.
Christ calls, the disciple follows; that is grace and commandment in one. “I will walk at liberty, for I seek thy commandments” (Psalm 119: 45).
from The Cost of Discipleship