The doctrine of our union with Christ (sometimes called the mystical union--not to be confused with the hypostatic union, which some also call the mystical union) is a powerful doctrine of Scripture that is difficult to teach. Essentially, it means that the Christian is truly united to Christ through the ministry of the Holy Spirit. This union is so real that the Christian actually shares in the body of Christ so that His body becomes our own. Christ also shares in our bodies so that our bodies are actually His own. This is how our sin became His sin and how His death and resurrection became our death and resurrection. This doctrine has rich implications, especially in regard to the sacraments. Even more, it has rich implications on how we live.
A few summers ago, I waded through Walter Marshall's The Gospel Mystery of Sanctification. I say "waded through" even though the book is small, because, like many Puritan classics, it was not easy reading for me. To make matters worse, the edition I read had terrible printing. The typeface was horrible and there were no margins. Just when I finished it, Bruce McRae released a new edition in modern English (with modern typeface). I haven't read the new edition, yet, but plan to this summer.
If I ever write a book (which I doubt I have the discipline to do), I would like to write a book loosely based on The Gospel Mystery of Sanctification, with stories and illustrations to connect with a modern audience.
Marshall's book could be called a celebration of our union with Christ. Here are some quotes from it. Unfortunately, these quotes are in the original English.
Men show themselves strangely forgetful, or hypocritical, in professing original sin in their prayers, catechisms, and confessions of faith; and yet urging on themselves and others the practice of the law, without the consideration of nay strengthening, enlivening means; as if there were no want of ability, but only of activity._______________
In order to be able to practice the law, we must have first want to do it, which means we must first be convinced that we are reconciled to God, and be convinced of our future enjoyment, and be convinced of sufficient strength to will and to do all our duties acceptably until we enjoy that future blessed state._______________
God has abundantly discovered to us, in His word, that His method in bringing men from sin to holiness of life, is, first to make them know that He loves them, and that their sins are blotted out._______________
In order to practice holiness, we must be persuaded of our future enjoyment of the everlasting heavenly happiness._______________
Though Christ is in heaven, and we on earth; yet He can join our souls and bodies to His at such a distance without any substantial change of either, by the same infinite Spirit dwelling in Him and us; and so our flesh will become His, when it is quickened by His Spirit;; and His flesh ours, as truly as if we ate His flesh and drank His blood._______________
The corrupt natural estate, which is called in scripture the old man, was crucified together with Christ, that the body of sin might be destroyed. And it is destroyed in us, not by any wounds that we ourselves can give it, but by our partaking of that freedom from it, and death to it, that is already wrought out for us by the death of Christ.
We are naturally so prone to ground our salvation on works, that if we cannot make them procuring conditions and causes of our salvation by Christ, yet, we shall endeavor at least to make them necessary preparatives, to fit us for receiving Christ and His salvation by faith.
Beggars will make most of their nasty rags, till they are furnished with better clothes; and cripples will not case away their crutches, until they have a better support to lean on.
Many have fallen into great sin because they do not account the grace of Christ sufficient for their pardon and salvation.
If Christians knew their own strength better, they would enterprise greater things for the glory of God.
There are many more. However, let me encourage you to read the book.