There are unions that are lawful and others that are not. Those who live according to the flesh unite with the flesh: "Do you not know that he who unites himself with a prostitute is one with her in body? For it is said, "The two will become one flesh" 1 Corinthians 6:16.
Instead those who live according to the spirit join the spirit: "But whoever is united with the Lord is one with him in spirit", 1 Corinthians 6:17.
The apostle adds: "Flee from sexual immorality. All other sins a person commits are outside the body, but whoever sins sexually, sins against their own body" 1 Corinthians 6:18.
The apostle explains here that the desires of the flesh and the desires of the spirit are opposed each other:
"For those who live according to the flesh set their minds on the things of the flesh; but those who live according to the spirit, set their minds on the things of the spirit. The mind governed by the flesh is death, but the mind governed by the spirit is life and peace. The mind governed by the flesh is hostile to God; it does not submit to God's law, nor can it do so." Romans 8: 5-7
Be one with the Lord in spirit it is first, to live according to the spiritual gift of rectitude, the golden rule: "Do unto others as you would have them do unto you", Matthew 7:12.
We have to choose between a life full of sins and a life of rectitude, between the flesh and the spirit, the conversion in the christian faith is the passage, the interior easter, from sin to righteousness: "No one can serve two masters. Either you will hate the one and love the other, or you will be devoted to the one and despise the other. You cannot serve both God and money." Matthew 6:24.
The union with God as such occurs in the spiritual birth or Gnosis. In the Gnosis we receive the essence of God, the wisdom, and thus receive the guidance of God and the fruits of the spirit: "When they arrived, they prayed for the new believers there that they might receive the Holy Spirit, because the Holy Spirit had not yet come on any of them; they had simply been baptized in the name of the Lord Jesus. Then Peter and John placed their hands on them, and they received the Holy Spirit" Acts 8:15
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