Church as the Body of Christ, as a sheepfold, as the kingdom of God, and as the kingdom of heaven. THE BIBLE SAYS...........
As the community of believers , the Church is the assembly (ekklesia) of all who believe in Jesus Christ; or the fellowship (koinonia) of all who are bound together by their common love for the Savior.
As the kingdom (basileia), it is the fulfillment of the ancient prophecies about the reign of the Messiah.
And as the Mystical Body it is the communion of all those made holy by the grace of Christ. He is their invisible head and they are his visible members. These include the faithful on earth, those in purgatory who are not yet fully purified, and the saints in heaven.
The Church is the congregation of all baptized persons united in the same true faith, the same sacrifice, and the same sacraments, under the authority of the Sovereign Pontiff and the bishops in communion with him.
Christ completed the founding of His Church just before His Ascension , when He commissioned the apostles to make disciples of all nations. Earlier in His public ministry He had instituted the sacraments, chosen the twelve apostles, instructed them by word and example, and conferred on them the power of teaching, ruling, and sanctifying.
Which is the one true Church established by Christ?
The one true Church established by Christ is the Catholic Church.
The Roman Catholic Church, officially known as the Catholic Church,    is the world's largest Christian church and represents over half of all Christians and one-sixth of the world's population .   It is made up of one Western church (the Latin Rite ) and 22 Eastern Catholic churches , divided into 2,782 jurisdictional areas around the world.  The Church looks to the Pope , currently Benedict XVI , as its highest human authority in matters of faith, morality and Church governance.   The Church community is composed of an ordained ministry and the laity .  Both groups may become members of religious communities such as the Dominicans , Carmelites and Jesuits . 
How do we know that the Catholic Church is the one true Church established by Christ?
We know that the Catholic Church is the one true Church established by Christ because it alone has the marks of the true Church.
The Catholic Church is one because all its members, according to the will of Christ, profess the same faith , have the same sacrifice and sacraments , and are united under one and the same visible head , the Pope.
The Catholic Church is holy because it was founded by Jesus Christ , who is all-holy, and because it teaches, according to the will of Christ, holy doctrines, and provides the means of leading a holy life, thereby giving holy members to every age.
Why is the Catholic Church catholic or universal?
The Catholic Church is catholic or universal because, destined to last for all time, it never fails to fulfill the divine commandment to teach all nations all the truths revealed by God.