Union with Christ perhaps the most significant theological concept of the Christian life that is the least taught, understood and believed.

Union with Christ is the linchpin of Paul’s teaching on sanctification. Below is a short but dense summary of union with Christ.  I’ve added Scripture references to enable me to not only think deeply about these awesome truths, but to ensure my thinking is not just informed by Scripture, but formed by it.  And though the truths are pervasive in Paul’s writing, I’ve concentrated on Colossians having just finished studying and teaching though this wonderful letter. (But I couldn’t help but add Romans 6!)

From John Murray in his Collected Writings [volume 2: Systematic Theology, page 289]:

“We are compelled to reach the conclusion that it is by virtue of our
having died with Christ,
and our being raised with him in his resurrection from the dead, {Colossians 2:11-15; Romans 6:1-4}
that the decisive breach with sin
in its power,
control,
and defilement
has been wrought,
and that the reason for this is that Christ
in his death
and resurrection {Colossians 3:1-4; Romans 6:3-11}
broke the power of sin, {Colossians 1:21-23; Romans 6:11}
triumphed over the god of this world, {Colossians 1:15}
the prince of darkness,
executed judgment upon the world and its ruler,
and by that victory
delivered all those who were united to him from the power of darkness, (Colossians 2:15)
and translated them into his own kingdom. {Colossians 1:13-14}
So intimate is the union between Christ and his people,
that they were partakers with him in all these triumphal achievements,
and therefore died to sin, {Romans 6:6-7, 11}
rose with Christ in the power of his resurrection, {Colossians 1:12}
and have their fruit unto holiness,
and the end everlasting life. {Colossians 1:21-23}
As the death and resurrection are central
in the whole process of redemptive accomplishment,
so are they central
in that by which sanctification itself is wrought in the hearts and lives of God’s people.” {Romans 6:12-14}