What was Paul trying to get across through his many "in Christ" and "in Him" passages?

Question

Paul uses "in Christ" and its variations many times in his epistles. What main idea(s) is he trying to convey with this phrase? Here is one example to consider:

2 Co 5:17 (KJV)

Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new.

Here are several other uses of the phrase in Romans (KJV), but you may focus your answer on the above to simplify:

Rom 8:1

There is therefore now no condemnation to them which are in Christ Jesus, who walk not after the flesh, but after the Spirit.

Rom 8:2

For the law of the Spirit of life in Christ Jesus hath made me free from the law of sin and death.

Rom 8:39

Nor height, nor depth, nor any other creature, shall be able to separate us from the love of God, which is in Christ Jesus our Lord.

My gut impression from reading these (and many other) "in Him" passages is that we in Christ Jesus are like being in a room in which God has stocked everything we'll need for life and godliness, everything we'll need to make it to the end in glory and honor. If we abide in Christ, we'll get the utmost in fruitfulness; our fruit will be tended to the fullest it can be by God. We can access any of them by faith. I'd love to know of any books written on this topic that are still readily available to the layman.

Answer 1

1 Corinthians 15:22 (DRB)

And as in Adam all die, so also in Christ all shall be made alive.

The fundamental idea is a radical identification with a progenitor: Adam the physical and spiritual progenitor of all mankind, Christ the spiritual—Christ the "[new] Adam" (1 Cor 15:45). Yet more than a progenitor.

We are born into the fallen version of our true nature, or as St. Paul says, we are "by nature children of wrath" (Eph 2:3).

St. Paul identifies Christ as the new Adam because, whereas we are by natural birth fallen, and in need of the grace which was proper to Adam before the fall, Christ provides a new spiritual birth, and allows us to derive our spiritual inheritance and lineage from Him instead.

Romans 5:12-19 (DRB)

Wherefore as by one man sin entered into this world, and by sin death; and so death passed upon all men, in whom all have sinned. 13 For until the law sin was in the world; but sin was not imputed, when the law was not. 14 But death reigned from Adam unto Moses, even over them also who have not sinned after the similitude of the transgression of Adam, who is a figure of him who was to come. 15 But not as the offence, so also the gift. For if by the offence of one, many died; much more the grace of God, and the gift, by the grace of one man, Jesus Christ, hath abounded unto many. 16 And not as it was by one sin, so also is the gift. For judgment indeed was by one unto condemnation; but grace is of many offences, unto justification. 17 For if by one man's offence death reigned through one; much more they who receive abundance of grace, and of the gift, and of justice, shall reign in life through one, Jesus Christ. 18 Therefore, as by the offence of one, unto all men to condemnation; so also by the justice of one, unto all men to justification of life. 19 For as by the disobedience of one man, many were made sinners; so also by the obedience of one, many shall be made just.

Cf. Wis 2:23-24; Ecclus 25:33; Isa 43:27.

There is also the separate usage employed by Jesus, although there is of course an overlap of meaning:

John 15:4 (DRB)

Abide in me, and I in you. As the branch cannot bear fruit of itself, unless it abide in the vine, so neither can you, unless you abide in me.