Sanctification is also known as becoming holy(Ephesians 1:4) and being conformed to His image(Romans 8:29). It cannot be left out of God's purpose because it is the step whereby we are transformed into the image of His Son, as well as into the image of the Father. It is in this step that we begin to take on the characteristics of the Family—where we begin to think and act like the current members of the Family of God. The character, the mindset, the attitudes, the perspective, the way we think, the way we look at things begins to become just like the God's.
Jesus says in Matthew 5:14, 16 that "a city that is set on a hill cannot be hidden. . . . Let your light so shine before men, that they may see your good works." Sanctification—if it is taking place in a person—cannot be hidden. Why is God so concerned about sanctification? Because 1) this is the step in His purpose in which the major portion of the transformation takes place, and 2) it can be seen—this is how we make a witness! Thus, when Paul sees the working faith, the laboring love, and the patient hope of the Thessalonians, he writes:
. . . remembering without ceasing your work of faith, labor of love, and patience of hope in our Lord Jesus Christ, in the sight of our God and Father, knowing, beloved brethren, your election by God. (I Thessalonians 1:3-4)
Seeing the fruits of their lives, he knew that they had been begotten by God—that they had God's Spirit—because they had begun "looking" like the Family. Therefore, if a person claims to be a son of God but habitually lives in sin—he is deceiving himself. Those qualities that identify his "spiritual ancestry" begin to show. "Family ties" can be seen.