One of the most crucial but difficult issue here is the meaning and function of Paul's mystical language (e. g. his "being in Christ mysticism"), which of course is somewhat shaped by cultural precedent.--language that pioneering scholar, Albert Schweitzer tries to describe in his seminal book on Paul's mysticism.
For Paul, the regenerating work of the Holy Spirit involves an experience of divine power that is publicly demonstrated and is not reducible to the wisdom of human analysis (1 Cor 2:4-5). The Spirit's initial work is also associated with the experience of "miracles" (Gal. 3:3, 5), but the limits of what Paul envisages by the term "miracles" is unclear. Does Paul have in mind the speaking in tongues that is part of the pattern for the initial reception of the Spirit in the Book of Acts? What is clear is this: Paul's concepts of walking in the Spirit and being led by the Spirit (e. g. Gal. 5:16-25) can be reduced to neither ethical conduct nor mere doctrine about what is simply a dry matter of faith.
The sense in which my ego is for Paul replaced by Christ's ego, again, is experiential language, which is hard to express in words:
"I have been crucified with Christ, and it is no longer I who live, but Christ who lives in me (Gal. 2:19-20)."
Similarly, the sense in which I an "a new creation" in Christ is crucially, but elusively experiential:
"If anyone is in Christ, he is a new creation: everything old has passed away; everything has become new (2 Cor. 5:17)."
The sense in which "everything old has passed away" has a mystical component that involves a new way of perceiving life and is experiential aspect shaped by cultural expectations that are very hard to express in words. This experiential element is not a once-for-all phenomenon, but rather reaquires daily experiential renewal:
"Even though our outer man is perishing, our inner man is being made new day by day (2 Cor. 5:16)."
In my view, these insights not only warn against efforts to reduce Paul's concepts to a fixed theological system of doctrine, but point to 1 Cor. 4:19-20 as one of the most important neglected keys to basic Christian experience:
"I will come to you soon, if the Lord wills, and I will find out not the [God] talk of these arrogant people, but their power. For the kingdom of God depends not on [God] talk but on power."