The oneness of the church
The doctrine of the "oneness" of the Godhead is perhaps the most foundational teaching in the Bible (Deut 6:4 with Mk 12:29, Mt 16:15-18). That there is only one true God, and that He is indivisibly one in His nature and existence, is essential to all other knowledge of Him. This doctrine has many aspects and great depth but can also be summarized quite simply. God is a Spirit (Jn 4:24). He is one Spirit (Eph 4:4). A spirit does not have flesh and bones (Lk 24:39). For the salvation of man, God manifested Himself in the flesh as Jesus Christ (Jn 1:1-3 & 14, Jn 14:7-11, 2Cor 5:19, Col 2:8-12, 1Tim 3:16). After Jesus ascended into heaven and was no longer on the earth in the flesh, He manifested Himself in His Spirit to men (Jn 7:37-39, Jn 14:16-28, Jn 16:5-15, Acts 2:1-4, Gal 4:6, Eph 2:18, Phil 1:19, 1Pet 1:11).
During what is called the last supper or the Lord's supper, Jesus said a prayer for His disciples. This was His last prayer before He headed to the garden to be betrayed and arrested. There are several important themes in this prayer but let us focus on one here. That is the word "one". It appears six times (vs. 11 & 21-23). It is the Greek word heis. Its basic meaning is the primary numeral one. There are examples of where this word is used in relation to God being absolutely one (Mt 23:8-10, Mk 10:18, Mk 12:29, Jn 10:30, Rom 3:30, 1Cor 8:4, Gal 3:20. Eph 4:5, 1Tim 2:5, Jam 2:19, Jam 4:12). Numerology can be a false mystical interpretation of scripture, but there are certain numbers which are emphasized in the Bible and seem to have certain significance such as seven. If there is a Biblical number for God, it is one. However, there are also other aspects of the word heis which the scriptures teach, and they are included in the prayer of Jesus in John 17. First, we must realize that Jesus did not pray as God in His divinity. That is confusion. Jesus is God manifested in the flesh, and as such He has both deity and humanity in one person. He prayed as a man (Ps 65:2, Heb 5:7). Some would say to this that it means Jesus prayed to Himself, but those who are filled with the Holy Ghost have God in them and they do not pray to themselves. They pray in their flesh to the God that is in them and is also everywhere else at once. That is not to make Christians equal with the Lord. He is unique and has no equal (Ex 8:10, Ex 9:14, Is 40:18, Is 46:5 & 9). As always, context is a key to proper
The first time Jesus used the word heis in John 17 is in verse 11 when He prayed, "Holy Father, keep through thine own name those whom thou hast given me, that they may be one, as we are". His desire is for Christians to be one. So, how can Christians be one with each other? Obviously, they cannot literally be numerically one, but they can be unified in their identity, purpose, and priorities. The word heis is used throughout the New Testament to describe this:
Jn 11:52 - gather together in one
Rom 12:4-5 - many members in one body (1Cor 12:11-26 - 13 times)
Rom 15:6 - one mind and one mouth
1Cor 3:8 - he that planteth and he that watereth are one
1Cor 6:17 - he that is joined unto the Lord is one spirit
1Cor 10:17 - For we being many are one bread, and one body: for we are all partakers of that
Gal 3:28 - ye are all one in Christ Jesus
Eph 2:14-15 - he is our peace, who hath made both one
Phil 2:2 - of one accord, of one mind
Col 3:15 - ye are called in one body
Acts 2:1 & 42-46 and Acts 4:32-35 also describe how unified the early church was. They were in one accord, one doctrine, and one fellowship, and they even shared their material goods to meet real needs. This was voluntary and private and has nothing to do with socialism which is mandated by the secular (usually atheistic) government. Acts 4:32 says they were so unified that they were of one heart and of one soul. Again, that is not numerically one. Another example of heis meaning of the same but not literally the same is Acts 17:26 which says God "hath made of one blood all nations of men for to dwell on all the face of the earth" in speaking of the brotherhood of man. Each person has their own blood in their body which is specific to them and differs in some ways from that of others, such as in DNA and blood type. Yet each of us has the same basic makeup of blood which makes us all one in humanity. We all bleed red as the saying goes. Romans 12 and 1Corinthians 12 speak of the church as parts of a body that have individual purposes but also work together for the greater purpose of the whole body. That is how Christians can be one with each other (heis).
Jesus also prayed for another aspect of the oneness of the church in John 17:21-23. He prayed:
vs 21 - That they all may be one; as thou, Father, art in me, and I in thee, that they also may
be one in us
vs 22 - that they may be one, even as we are one
vs 23 - I in them, and thou in me, that they may be made perfect in one
The purpose of this is so that:
vs 21 - that the world may believe that thou hast sent me
vs 23 - that the world may know that thou hast sent me
So, if God is absolutely one, how can the church be one as He is? The key to these statements is the same key to understanding all verses on the Godhead - the dual nature of Jesus Christ. When we read verses about such things as the Father and the Son, we should know that the Bible does not teach that there are three separate and distinct persons in the Godhead. Rather, there are two natures in one person - Jesus Christ. In Him is both divine and human, God and man, Spirit and flesh, Father and Son. For example, Jesus was asleep in a boat but then arose and rebuked the storm (Mt 8:23-26). He was asleep as a man (Ps 121:4) but took control of nature as its God and creator (Jn 1:3 & 10, Col 1:16). When people become Christians, they do not cease to be human; but they do become partakers of the divine nature through the indwelling of His Spirit (2Pet 1:3-4). As the Son submitted to the will of the Father and did and said what He wanted (Lk 22:42, Jn 5:17-36, Jn 8:28-29 & 38, Jn 10:18 & 37, Jn 12:49-50, Jn 14:31, Jn 15:15), so He empowers His children to do likewise. So the Father was in the Son by His Spirit which was in Him, and the Son was in the Father by relationship and because the Son always did the will of the Father. Thus, there was a reciprocal indwelling (Jn 14:20). When someone receives the gift of the Holy Ghost and does God's will, they too become a partaker of that relationship, indwelling, and unity. They do not become God. That is the doctrine of Satan (Gen 3:5, Mt 24:5 & 23-27, 2Thes 2:1-12). They enter into a relationship with the Father as adopted children (Rom 8:15 & 23, Gal 4:4-7, Eph 1:5). Jesus was the only begotten Son of God (Jn 1:14-18, Jn 3:16-18, Heb 11:17), but through Him we can become the adopted children of God (Jn 1:12). We can be born again of water and of the Spirit (Jn 3:3-8) through repentance, water baptism by immersion in His name for the remission of sins, and receiving the gift of the Holy Ghost (Acts 2:38, Acts 8:12-17, Acts 10:43-48, Acts 19:1-6).
We should not be troubled by the use of the word "us" in John 17:21. This has nothing to do with multiple divine persons, but rather the two natures which are in Christ. First, remember that this is within a prayer of Jesus, so this is humanity speaking to divinity. Second, there are other aspects to the use of the words us and we such as in Genesis 1:26, Genesis 3:22, and Genesis 11:7 which upon study show us that it is the one God speaking and not multiple persons speaking in unison or conversation. It is God consulting with His own will (Eph 1:11). People also do this. For example, have you ever spoken to yourself and said, "Let's see"? It can also be used when one is speaking on behalf of a group (Ezra 4:18).
Christians can be one with God by being born again and following His will, and they can be one with each other by being unified in God's purpose for the church. This is what Jesus prayed for, and may His prayer be answered, so that God will be glorified and people's faith will be placed in the Lord.