1 Corinthians 1:30 (NIV): It is because of him that you are in Christ Jesus, who has become for us wisdom from God—that is, our righteousness, holiness and redemption.
Union with Christ is an important Biblical theme: it concerns the Biblical idea that we are in some way united to Jesus Christ, and is expressed in the New Testament through the phrases "in Christ" and "into Christ". These phrases occur over 200 times just in Paul's letters, and is a major theme of John's Gospel, let alone the rest of the NT.
Union with Christ is fundamental to the Reformed doctrine of salvation, our union with Christ being what applies Christ's redeeming work to us as individuals. But I suspect that in other denominations and theological traditions Union with Christ plays a different role.
What is an overview of the understanding and role of Union with Christ across Christianity? Some denominations/theological schools may not use the term "Union with Christ", but due to the high occurrence of "in Christ" they will surely have some explanation of what these phrases mean.
Note that this is an overview question: answers must summarise the positions of all major Christian branches, and if possible some of the smaller ones as well.