It is Christ's past
...what Paul is saying is that sanctification means this: in relationship both to sin and to God, the determining factor of my existence is no longer my past. It is Christ's past. The basic framework of my new existence in Christ is that I have become a "dead man brought to life" and must think of myself in those terms: dead to sin and alive to God in union with Jesus Christ our Lord .
Sinclair Ferguson, "The Reformed View", in Donald Alexander, ed., Christian Spirituality: Five Views on Sanctification (Downers Grove, IL: IVP Academic, 1988), 57