I therefore, a prisoner for the Lord, urge you to walk in a manner worthy of the calling to which you have been called, with all humility and gentleness, with patience, bearing with one another in love, eager to maintain the unity of the Spirit in the bond of peace.

There is

• one body and one Spirit (just as you were called to the one hope that belongs to your call)
• one Lord,
• one faith,
• one baptism,
• one God and Father of all, who is over all and through all and in all.

But grace was given to each one of us according to the measure of Christ’s gift.

...

And he gave the apostles, the prophets, the evangelists, the shepherds and teachers, to equip the saints for the work of ministry, for building up the body of Christ, until we all attain to the unity of the faith and of the knowledge of the Son of God, to mature manhood, to the measure of the stature of the fullness of Christ, so that we may no longer be children, tossed to and fro by the waves and carried about by every wind of doctrine, by human cunning, by craftiness in deceitful schemes.

Rather, speaking the truth in love, we are to grow up in every way into him who is the head, into Christ, from whom the whole body, joined and held together by every joint with which it is equipped, when each part is working properly, makes the body grow so that it builds itself up in love.

-- Ephesians 4:1-7,11-16 (ESV)

For example when Saul persecuted Jesus' disciples, Jesus said Saul was persecuted him:

But Saul, still breathing threats and murder against the disciples of the Lord, went to the high priest and asked him for letters eto the synagogues at Damascus, so that if he found any belonging to the Way, men or women, he might bring them bound to Jerusalem. Now as he went on his way, he approached Damascus, and suddenly a light from heaven shone around him.

And falling to the ground, he heard a voice saying to him, “Saul, Saul, why are you persecuting me?”
And he said, “Who are you, Lord?”
And he said, “I am Jesus, whom you are persecuting. But rise and enter the city, and you will be told what you are to do.”

-- Acts 9:1-6 (ESV)

Paul wrote that Jesus' disciples are "joined together" with Jesus into "one body":

For he himself is our peace, who has made us both one and has broken down in his flesh the dividing wall of hostility by abolishing the law of commandments expressed in ordinances, that he might create in himself one new man in place of the two, so making peace, and might reconcile us both to God in one body through the cross, thereby killing the hostility. And he came and preached peace to you who were far off and peace to those who were near. For through him we both have access in one Spirit to the Father. So then you are no longer strangers and aliens, but you are fellow citizens with the saints and members of the household of God, built on the foundation of the apostles and prophets, Christ Jesus himself being the cornerstone, in whom the whole structure, being joined together, grows into a holy temple in the Lord. In him you also are being built together into a dwelling place for God by the Spirit. -- Ephesians 2:14-22 (ESV)

Jesus said that when one serves with a prophet or righteous person, it is rewarded as if one was serving Jesus himself.

“Whoever receives you receives me, and whoever receives me receives him who sent me. The one who receives a prophet because he is a prophet will receive a prophet’s reward, and the one who receives a righteous person because he is a righteous person will receive a righteous person’s reward. And whoever gives one of these little ones even a cup of cold water because he is a disciple, truly, I say to you, he will by no means lose his reward.” -- Matthew 10:40-42 (ESV)

This is also confirmed a few chapters later when Jesus said:

"For I was hungry and you gave me food, I was thirsty and you gave me drink, I was a stranger and you welcomed me, I was naked and you clothed me, I was sick and you visited me, I was in prison and you came to me."
Then the righteous will answer him, saying, "Lord, when did we see you hungry and feed you, or thirsty and give you drink? And when did we see you a stranger and welcome you, or naked and clothe you? And when did we see you sick or in prison and visit you?"
And the King will answer them, "Truly, I say to you, as you did it to one of the least of these my brothers, you did it to me."

-- Matthew 25:35-40 (ESV)

Which explains why Mark recorded that Jesus said:

For truly, I say to you, whoever gives you a cup of water to drink because you belong to Christ will by no means lose his reward. -- Mark 9:41 (ESV)

How the body is joined

For just as the body is one and has many members, and all the members of the body, though many, are one body, so it is with Christ. For in one Spirit we were all baptized into one body — Jews or Greeks, slaves or free — and all were made to drink of one Spirit. For the body does not consist of one member but of many.

If the foot should say, “Because I am not a hand, I do not belong to the body,” that would not make it any less a part of the body. And if the ear should say, “Because I am not an eye, I do not belong to the body,” that would not make it any less a part of the body. If the whole body were an eye, where would be the sense of hearing? If the whole body were an ear, where would be the sense of smell? But as it is, God arranged the members in the body, each one of them, as He chose.

If all were a single member, where would the body be?

As it is, there are many parts, yet one body. The eye cannot say to the hand, “I have no need of you,” nor again the head to the feet, “I have no need of you.” On the contrary, the parts of the body that seem to be weaker are indispensable, and on those parts of the body that we think less honorable we bestow the greater honor, and our unpresentable parts are treated with greater modesty, which our more presentable parts do not require. But God has so composed the body, giving greater honor to the part that lacked it, that there may be no division in the body, but that the members may have the same care for one another.

If one member suffers, all suffer together; if one member is honored, all rejoice together.

Now you are the body of Christ and individually members of it. And God has appointed in the church congregation first apostles, second prophets, third teachers, then miracles, then gifts of healing, helping, administrating, and various kinds of tongues. Are all apostles? Are all prophets? Are all teachers? Do all work miracles? Do all possess gifts of healing? Do all speak with tongues? Do all interpret? But earnestly desire the higher gifts.

-- 1 Corinthians 12:12-31 (ESV)

(Translators often confuse "church" with "the body of Christ", the original Greek word "ekklésia" translates to "congregation")

As Paul pointed out, the body is not a passive fan club, but each member should actively participate which his or her given gifts, talents or skills to support the body.

For as in one body we have many members, and the members do not all have the same function, so we, though many, are one body in Christ, and individually members one of another. Having gifts that differ according to the grace given to us, let us use them:

• if prophecy, in proportion to our faith;
• if service, in our serving;
• the one who teaches, in his teaching;
• the one who exhorts, in his exhortation;
• the one who contributes, in generosity;
• the one who leads, with zeal;
• the one who does acts of mercy, with cheerfulness.

-- Romans 12:4-8 (ESV)

Just as any organic body needs nourishment to stay alive, so does the believers themselves need support. In a practical sense this depends on the current needs and could include anything like accountability, rendering basic services (skilled or unskilled), teaching each other, sharing testimonies, celebrating together, providing physical resources (like food, clothes, shelter, etc.), organizing gatherings, as well as prayer and encouragement.

For example:

And they devoted themselves to the apostles’ teaching and the fellowship to the breaking of bread and the prayers. And awe came upon every soul, and many wonders and signs were being done through the apostles. And all who believed were together and had all things in common. And they were selling their possessions and belongings and distributing the proceeds to all, as any had need. And day by day, attending the temple together and breaking bread in their homes, they received their food with glad and generous hearts, praising God and having favor with all the people. And the Lord added to their number day by day those who were being saved. Acts 2:42-47 (ESV)

This is perhaps an extreme example of how close the first congregation's fellowship were.

The responsibilities of the body of Christ has been handed over to various church institutions with the result that Christians are spiritually dying as they receive little to no nourishment because everyone is mainly focussed to support the church leadership and their agendas instead of nourishing the body of Christ and spreading the Gospel.

And let the peace of Christ rule in your hearts, to which indeed you were called in one body. And be thankful. -- Colossians 3:15 (ESV)

When one eat or drink with other, you fellowship with those persons. This is the same reason why friend or lovers would go out on a date to eat or drink together. This is also the same reason why a spouse would be jealous when he/she finds his/her partner eating or drinking with someone else. Therefore, when we are having fellowship with pagans (eat and drink) we provoke the Lord to jealousy.

Therefore, my beloved, flee from idolatry. I speak as to sensible people; judge for yourselves what I say.

The cup of blessing that we bless, is it not a participation in the blood of Christ?
The bread that we break, is it not a participation in the body of Christ?

Because there is one bread, we who are many are one body, for we all partake of the one bread.

Consider the people of Israel: are not those who eat the sacrifices participants in the altar? What do I imply then? That food offered to idols is anything, or that an idol is anything?

No, I imply that what pagans sacrifice they offer to demons and not to God. I do not want you to be participants with demons. You cannot drink the cup of the Lord and the cup of demons. You cannot partake of the table of the Lord and the table of demons.

Shall we provoke the Lord to jealousy? Are we stronger than He?

-- 1 Corinthians 10:14-22 (ESV)

It was never about what we eat and what we drink, but about with whom we are having fellowship for what purpose.

Therefore let no one pass judgment on you in questions of food and drink, or with regard to a festival or a new moon or a Sabbath. These are a shadow of the things to come, but the substance belongs to Christ. Let no one disqualify you, insisting on asceticism and worship of angels, going on in detail about visions, puffed up without reason by his sensuous mind, and not holding fast to the Head, from whom the whole body, nourished and knit together through its joints and ligaments, grows with a growth that is from God. -- Colossians 2:16-19 (ESV)

Therefore, the body of Christ, are the believers actively participating in fellowship with their gifts, to nourish each other with the aim to grow the body in the Gospel of Jesus Christ.