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John C said:

I just don't have time presently to listen to the audio. Can you give a summary of what is being said.

Basically, Rev. Garcia argues that for Calvin, union with Christ is the overarching context in which justification and sanctification (the duplex gratia or "twofold grace") are related. It is in union with Christ that we receive all of the benefits of Christ's mediatorial work. In this way justification and sanctification both retain their proper distinction from and relation to one another. Both flow from the Person of Christ, both are the necessary result of being joined to Him.

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I don't really think of being in union in Christ. I've been in Reformed Christian circles for over 20 years and until recently I hadn't heard much said about union. It is easier for me to grasp that I am adopted. Are the two saying the same idea or are they different?

They are different. Union with Christ is being joined to the Son as our head and husband, whereas adoption is where the Father places His Name on us and grants us the status of sons of God. See the Larger Catechism:

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Q66: What is that union which the elect have with Christ?
A66: The union which the elect have with Christ is the work of God's grace, whereby they are spiritually and mystically, yet really and inseparably, joined to Christ as their head and husband; which is done in their effectual calling.

Q74: What is adoption?
A74: Adoption is an act of the free grace of God, in and for his only Son Jesus Christ, whereby all those that are justified are received into the number of his children, have his name put upon them, the Spirit of his Son given to them, are under his fatherly care and dispensations, admitted to all the liberties and privileges of the sons of God, made heirs of all the promises, and fellow heirs with Christ in glory.

Kyle

I tell you, this man went down to his house justified.