Sanctification is the process of being set apart for God's work and being conformed to the image of Christ.  Sanctification is not instantaneous because it is not the work of God alone. The justified person is actively involved in submitting to God's will, resisting sin, seeking holiness, and working to be more godly If we live in the Spirit, let us also walk in the Spirit (Galatians 5:25).
 

Unto the church of God which is at Corinth, to them that are sanctified in Christ Jesus, called us to be saints, with all that in every place call upon the name of Jesus Christ our Lord, both theirs and ours (KJV 1 Corinthians 1:2).