To us, just as to you, the explanation you provide seems strange. The Apostle Paul, as far as can be understood, is speaking about the following...
To us, just as to you, the explanation you provide seems strange. The Apostle Paul, as far as can be understood, is speaking about the following. He who unites himself with a prostitute is one with her in body. Conversely, the apostle writes, he who unites himself with the Lord is one with Him in spirit. The apostle uses the verb kollao in both cases, literally meaning "to glue, to cleave, to join." He is appealing to the imagery conveyed in the Book of Genesis by the words "and a man shall cleave to his wife, and they shall become one flesh." This is where the verb "cleave" comes from.
Just as the words "they shall become one flesh" do not imply merging, so in the second case the bold imagery of the apostle does not imply the merging of the human spirit with God. On the contrary, the apostle is faithful to the tradition of the Old Testament, which uses marriage as an image of the relationship with God, where there is essential unity, the closest of bonds, while preserving the distinction of persons, where the persons do not merge into one, but become inextricably together. In the union of people "in one spirit with the Lord" there is similarly no merging, the essential distinction of individualities similarly remains, but the closeness of this unity is such that it is impossible to imagine one without the Other.
Thus, it refers to what the theology of the first millennium (so-called patristic theology) termed "theosis" (deification). The Lord Jesus Christ spoke of this: "that all of them may be one, Father, just as you are in me and I am in you, may they also be one in us." This is the entering of people into the mystery of the love of the Trinity, such love that overcomes difference and unites without violating freedom and individuality. Just as the Father and the Son are one, so too can a person who has cleaved to the Lord be one with God and other people. Strictly speaking, this unity without merging is paradoxical; human thought knows no examples of it other than the Trinity. And such union with God is precisely the ultimate goal of our lives.
Therefore, the Apostle Paul is speaking in a somewhat unusual manner about the covenant (union) with God, to which humanity is called.