It is any church other than the original that Christ built. It can have any name, but the most obvious ones are those that are widespread and well known (Mormon, Lutheran, Baptist, Adventist, Pentecostal, Catholic, etc.) The original church was the Church of God - Christ's Church, the Lord's Church. The church of Christ is always used in a descriptive way; it identifies the people of God. We are the church of Christ.
By definition, a denomination is a collective of congregations. When congregations function as a unit, they function denominationally. So, it is more than just something named. "Church" is a called-out group of people. The "church of Christ" then is Christ's called-out people from the world to Him. It is God's group of people.
WHAT CHANGES HAVE BEEN MADE?
You name it, someone has changed it.
They preach that you can get baptized by sprinkling instead of immersion, you can get baptized if you want to or not if you don't want to, being baptized as a baby is good enough, or it is a sin to think that baptism has anything to do with salvation.
They preach that Jesus is a lesser God because He is only the Son, that Jesus stopped being part of the Godhead when He came to earth as a man, Jesus never was part of the Godhead, etc.
They added a piano, they added an organ, they added a piano and a harp, they added guitars, or they added electric guitars and drums.
They sing then the choir sings, they don't sing but let the choir do all the singing since they sound better, they just grunt while kissing snakes, or they just moan as they roll in the aisles.
They pray to saints, they pray to the mother of Jesus, they pray to a man who calls himself Christ on earth and others call the pope, they pray to pictures of Jesus, they pray to a cross, or they pray to a necklace of beads.
They invent higher titles to strive for like cardinal, "Lord Pope," the pontiff, "Holy Father," archbishop, auxiliary bishop, coadjutor, abbott, or padre.
They preach that God wouldn't let good people go to hell, that there is no hell, that there is no Heaven, that hell is just not being in Heaven instead of being a lake of fire, that dead is dead, or that the living can pray the dead out of hell and into Heaven.
What the changes, additions or deletions are is irrelevant. God has a perfect plan, and Jesus built His church according to that plan. We either put ourselves above God or we join hands with the devil when we make changes to His church. You alone are responsible now for your choice of church. It doesn't matter who changed it or how long ago it was changed.
In the beginning, there was belief in God - the one God. This was the main theme of Judaism. The Jews were God's chosen people, and Jesus was Jewish. Almost from the start, people started inventing other "gods" to worship instead (ROMANS 1:18-25) - maybe these invented "gods" had fewer rules and allowed for lustful diversions like adultery and bestiality. When God sent his only begotten Son to die for our sins, Jesus became the head of the new church - the one church. We Christians became his saints, the body of the Christ (with Jesus as the head of the church), and his new chosen people in his new covenant. And, many Jews were added to the new church.
So that the law was our custodian until Christ came, that we might be justified by faith. But now that faith has come, we are no longer under a custodian; for in Christ Jesus you are all sons of God, through faith. For as many of you as were baptized into Christ have put on Christ. There is neither Jew nor Greek, there is neither slave nor free, there is neither male nor female; for you are all one in Christ Jesus. And if you are Christ's, then you are Abraham's offspring, heirs according to promise.
Rome was the center of the world, and the Roman leaders under Bonifice III tried to take control of the church. This started the Roman Catholic religion, in 606 A.D. Some ruler long ago wanted to get baptized before he died but could not physically make it to the immersion bath, so the priests brought a little water to him. This was the beginning of the end of the one baptism taught by Jesus Christ. After all, it was supposedly good enough for him to get into Heaven so others demanded it. Could the priests admit that they lied to the dying ruler? To save face, they allowed the change. But, this was just one of the many new doctrines they invented, as prophesied as "the pretensions of liars whose consciences are seared, who forbid marriage and enjoin abstinence from foods which God created to be received with thanksgiving by those who believe and know the truth." (1 TIMOTHY 4:1-5)
Martin Luther was a Catholic monk. He was appalled at all he read in the Bible and how it differed from what the priests were told to teach their members. The people didn't know any better because only the clergy were allowed to read the Bible. Martin Luther left the Roman Catholic religion and started a reformation of Catholicism. He became well loved for taking such a risk to his own life, so he made people around him swear that they would not call themselves Lutherans when he died. But they did. The Lutheran religion began in 1520 A.D. Although Luther improved on the Catholic doctrine, he did not return to Christ's doctrine.
King Henry VIII decided that he would create his own religion in 1534 A.D. (Episcopalian) when the Catholic clergy wouldn't allow him to marry Anne Boleyn. He and his religion were the main reason that America was settled, since he made it law that his religion be the only allowed one in England. (That is why Separation of Church and State developed in America - so we wouldn't have dictated to us how we could worship God. Or you could say so that Government would be kept out of religion - not so that God was not allowed in Government - then schools - etc.)
Other denominations were formed (such as Presbyterian, Baptist, Methodist, Mormon, Adventists, Christian Scientist, and Jehovah Witness), many of which were invented by men as late as the 1800s.
All of these religions were invented by men, many have some Christian characteristics, and many also have some Catholic characteristics. Only plain old Christian is allowed and sanctified by God. When Jesus died for all of us, Judaism was no longer sanctified by God. Jesus Christ's was the beginning of Christianity. All of the Jews should have followed, and anyone who obeys what God says in the New Testament needs to give up manmade religion and simply follow God's Word.
"Everything that I command you you shall be careful to do; you shall not add to it or take from it."
The problem is not just names besides "Christian" ("Catholic," "Lutheran," "Baptist," etc.), it is also anywhere new "doctrines" were added by these people and God's doctrine was changed by these people. Even a lot of the most basic churches that profess to be just plain old Christian have changed, added to or taken away from how God said we should worship.
"Truly, truly, I say to you, if any one keeps my word, he will never see death."
Jesus answered him, "If a man loves me, he will keep my word, and my Father will love him, and we will come to him and make our home with him."
But he said, "Blessed rather are those who hear the word of God and keep it!"
God does not tell us that we should obey part of His Word, or that we should obey the parts of the Word of God that we like and ignore the parts we don't like. It is the perverse doctrine of men that tells some that baptism is not really necessary as long as they believe in Jesus as their Savior, or that tells others they should be baptized but it doesn't matter how it is done, or that tells still others they should be baptized but they never have to be good or repent when they sin. (Although the entire Bible is the God's Word, the Old Testament refers to the Word of God as the law or commandments, which are the law of Moses, and the Word of the LORD when God spoke in one form or another. The New Testament refers to the Word of God through Jesus Christ instead of through the law. We must follow all of the New Testament (not just the parts we like), while not forgetting God's commandments as rules to live by. Since God made a new covenant with us through Jesus instead of law, we are no longer required to keep to the letter of the Old Testament law (like sacrificing bulls). In fact, keeping to the letter of the law without a good heart or good works is worthless.)
Before we are saved, we are all imperfect, unholy sinners. God is perfect and holy, and God can't be close to sin. Jesus is our only link to God. We are able to be made justified if we are washed with the blood of Jesus through baptism. Those who believe in God, repent from their old ways, confess Jesus Christ as Lord, get baptized correctly, obey God's Word (and not the doctrines of men), pray to God in the name of Jesus Christ, do good works, and do all of this in truth and spirit will be saved.
"Enter by the narrow gate; for the gate is wide and the way is easy, that leads to destruction, and those who enter by it are many. For the gate is narrow and the way is hard, that leads to life, and those who find it are few."
God didn't say it would be easy. But, eternal life later and a wondrous life in fellowship with God until then surely seems worth it! When you love God, it is not too hard to do what pleases him but is hard to hurt him on purpose. When you love Jesus, it is not too hard to follow his ways, especially if you submit to him and let him lead the way. When you love God and Jesus, it is not a chore to go to church and Bible study - these are just more opportunities to worship Almighty God, thank Jesus for all His sacrifices, and to learn more of God's entire wondrous plan.