What is Union with Christ
Sanctification is therefore neither self-induced nor created in us by divine fiat. Like justification, it has to be "earthed" in our world (that is, in Christ's work for us in history) if it is to be more than a legal fiction. To change the metaphor, we can only draw on resources which have already been deposited in our name in the bank. But the whole of Christ's life, death, resurrection and exaltation have, by God's gracious design, provided the living deposit of his sanctified life, from which all our needs can be supplied. Because of our fellowship (union) with him we come to share his resources. That is why he can "become for us" sanctification, just as he is also our wisdom, righteousness and redemption (1 Cor 1:30)
Sinclair Ferguson, "The Reformed View", in Donald Alexander, ed., Christian Spirituality: Five Views on Sanctification (Downers Grove, IL: IVP Academic, 1988), 58