Why do we say, "The church instituted by Jesus is one"?

Why do we say,  "The church instituted by Jesus is
one"?
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Catholic church is a Divine institution,

which means,  that

it was instituted by God Himself.

There is only one God.

The church of God is also one,

because,  its source is unity.

It was born in the mind of the

Most Holy Trinity, one God in three Persons.

Jesus Himself prayed to His Father :

"...that they may all be one;

that they too may be one in us,

as thou, Father, art in me, and I in thee;

so that the world may come to believe

that it is thou who hast sent me.. "(John.17:21)

Having its source in the Trinitarian unity,

the Church  must be one,

unified

within itself

under one head.

There must be unity within itself,

which is possible only if it is one Head.

By 'unity within itself', I refer to the unity in Faith,  which is the essence of our Church.

The Catholic Church  professes the same  faith that has been passed down from the Apostles (what we Catholics refer to as the deposit of faith).

If anyone breaks away from the Faith which belongs to the Apostolic tradition,  clinging to his own opinions, he stops belonging to the true Church.

Church’s oneness or unity is not created or brought about by us, 

but instead comes from God Himself.

Jesus instituted the Church in accordance with the will of Heavenly Father.

The Church is one

because of its founder:

"The Word made flesh,

the prince of peace,

reconciled all men to God by the cross, . . .

restoring the unity of all in one people and one body." (CCC 813)

The Holy Spirit is the soul of the Mystical body (Church)  instituted by Jesus.

One soul can have only one body.

One Holy Spirit can have only one Church.

How can the same Spirit dwell in more than one Church, giving them opposite inspirations? 

On the Pentecost day the Holy Spirit inspired the Apostles to preach the same Gospel to different people speaking different languages.

So the Church which has unbroken Apostolic tradition till now is the One true Church.

The Church is one in its celebration of  the seven sacraments that were instituted by Christ, especially the Eucharist.

Christ takes care of His Church  through His representative on the earth, the Pope.

So,  only the Church headed by the Pope is the true Church,  instituted by Jesus.

Through the sacrament of Holy Orders,

the Church’s apostolic succession

ensures uninterrupted continuity,

with the teaching and leadership of St. Peter
(the first  pope)

and

the Apostles (the bishops)

in union with him.

We cannot deny the fact

that  there have always been tensions

within the body of Christ,

the Church.

One need only read the Epistles of St. Paul to understand

that our sins have injured this unity from the very beginning.

In fact, "in this one and only Church of God

from its very beginnings there arose certain rifts,

which the Apostle strongly censured as damnable.

Around the period of Renaissance, 

some Church members ignored the oneness of the Church

and

under the pretext of Reformation

broke themselves away from the Church

for selfish motives,

taking with them only the Bible,

that too by ignoring certain books,

that didn't support their motives,

not accepting certain articles of Faith

and

rejecting Papal
Headship and tradition.

Now they are not within the one Apostolic Church,

instituted by Jesus.

They must return to the Catholic Church,

accepting the the authority of the Pope

and

the unbroken Apostolic tradition.

That is the will of God, revealed through the prayer of Jesus,

"I pray for those who are to find faith in me through their word;

21 that they may all be one; "
(John17:20,21)

Let God's will be done.

"Our Father,  Thy will be done on earth as it is done in Heaven."

Lourdu Selvam.