There is one body and one Spirit, just as you too were called to the one hope of your calling,
To highlight what he is saying about keeping the unity of the Spirit in the bond of peace, Paul wants to further impress on us the concept of oneness.
One body
There is nothing more “one” than the body. Every part of the body is affected by every part of the body because it is, by definition, a unified whole. We are one by default. It is one body of Christ; our oneness is defined by the head, Christ. There are no two Christs; therefore, there are no two bodies of Christ.
There has to be one way of becoming Christians, which is to believe in the Lord Jesus (see Acts 16:31; Romans 10:9–10). We are one with those who hold that to be the truth and can never be one with those who adopt another tenet.
One body means we are in close contact, close fellowship, we are connected, even if we are in different places. We feel closer to Christians in far-flung places of the earth than to a random person in our city or even to our family members in some cases.
One Spirit
We hold to the fact that there is one Spirit. That the Spirit in me is the Spirit in you, and that Spirit testifies with our spirit that we are children of God (Romans 8:16). There is no diversity of the Spirit of God.
A place where there are multitudes of spirits floating around is a place where we cannot be one with. That is animism, where there is a spirit of the river and the spirit of the rock, etc, and there is a superior spirit on the pastor and another lesser spirit on the members. We cannot be one with people in those places.
One hope
This is important. Hope tells us where we are going.
If we have ideas about where we are ending that are not founded in Christ,
where we have imagined ideas which say certain denominations will have special locations in heaven,
as if your hope therefore has to be in that denomination or group to get you to heaven,
with the leader as my means of getting to God and not Christ,
then we have a problem,
If you are going to direct my heart to hope in a system rather than in Christ, then we have a problem; we cannot be one.
The one hope of our calling also talks about lifestyle, motivation and destination: what we are doing, why we are doing it and where we are going (what we are aiming for).
We need to emphasize that and keep reemphasizing it - keeping the hope of our calling before us.
It is in that context that we will understand why Paul cried as he gave his final charge to the Ephesian church elders in the book of Acts (Acts 20:17–38).
He told them, with tears, that after he was gone, fierce wolves would come from among them, teaching perversions of the truth to draw disciples after them.
They might be carving for themselves:
alternative bodies where headship/lordship is self-derived
alternative spirits with probably principles derived from the world or false spirits derived from hell in false teaching
alternative hopes where the end we are aiming for is the same riches of this world that Satan promised Jesus (Matthew 4:8–9; Luke 4:5–7), including political domination etc.
It is the same church that Paul wrote this letter to after telling them he was sure some people would preach perverse things; preach exclusive truths, things not derived from the truth of the word of God.
And to that church, he said he entrusts them to God and the message of His grace, which is able to build them up and give them an inheritance among all those who are sanctified. That is the hope.
Paul did not say hope in me.
I, therefore, conclude that:
Placing hope in anything, any organization (apart from Christ and what He has done), is evil and demonic
Having a separate hope for you and your ingroup is evil and demonic.
Saying people in this denomination/church group, etc, will have a special place in heaven on account of our founder/man of God is evil and demonic
Making the above a selling point of your organization, why everyone must join and never leave, is evil and demonic.
There is no graduating from one body, one spirit and one hope.
Paul said some people will draw disciples after themselves. That is making themselves the center of things, the endpoint, rather than as a means to an end, which is Christ, His offering of eternal salvation and reward, and His coming kingdom.
Our oneness is
not a forced oneness
not a oneness on the barrel of the gun
not the oneness because of ethnicity and nationality or language or skin color
not the oneness tied to social standing or educational achievement or even age or experience
not a oneness tied to gender or occupation or aspiration
not a oneness based on history
But it is one body (one way of coming to Christ), one spirit (one divine operation within us), and one hope (one direction we are all aiming for).