The prayer that Jesus prayed that is recorded in John 17 speaks of union. He said, “Father, I am leaving the world, but these are staying and I am coming to you. Holy Father, keep through your own name those that you have given me, that they may be one, as we are one…neither do I pray for these only, but for all who will believe on me through their word; that they may all be one; as you Father, are in me and I am in you, that they also may be one in us: that the world may believe that you have sent me. And the glory that you have given me, I have given them, that they may all be one, even as we are one”.
This prayer is usually interpreted to be a request that Christians would cooperate with one another, and agree with one another, thus presenting a unified front to the world, that the world may believe that God sent Jesus.
Although this is a good application of the verse Jesus seems to be also asking that Christians could be in Him and in the Father, as He is in the Father and the Father is in Him, that the Christian could be one with them. That is, have union with the Godhead.
Now the uniform testimony of those practiced in inward prayer, both Catholic and Protestant, is that such a union became their experience. That, in fact, the goal of inward prayer is union.
The union of the Christian spirit and the Godhead is experienced as an intimate closeness with God, characterized by a union of will and a peace “that passes understanding”. The person feels that they are one with God. They are quick to clarify, though, that they do not believe they have become God. This is a perception of union, where the person feels one with God; they realize they are still a human, a finite spiritual being, who began life when they were born on earth, not divine or eternal. This is a spiritual union not a union of identity. Often the spiritual union is expressed as a spiritual marriage, or espousal to the Lord.
The main characteristics they describe are a union of the will and a deep abiding peace. They no longer feel that their will is contrary to God’s will, they have become of one will. Likewise, they no longer feel separate from God or suffer the agitation of independence. The union of will and the rest they feel make their lives fruitful and productive. They know what God wants and they fulfill His purpose by gladly following His Spirit.
Please realize that this state is not a constant state of peace and bliss. Nor is a onetime experience; it is a progressive process.
Other perceptions are also reported, mainly a sense of being one with nature or being one with all humans. These can be felt intensely with a conviction of actual oneness. To some all things and all people seem connected in the One.
Some errors can arise from these perceptions and reports of these experiences, most notably a conviction that God sees all people equally and that all humans will end up in Heaven with God. This tendency towards Universalism has been strengthened recently by secular globalization, post-modern quantum physics, a desire to be tolerant/inclusive and a distancing of Christians from the cultural abuses of historic Christianity. The misuse of the experience of Union is therefore a current challenge to those who are disillusioned with historic Christianity and who are exploring emergent Christianity.
See also The Eternal Now.