What is religion?

As a Christian what role does religion have? Why are there so many denominations and religions?

Merriam-Webster's dictionary defines religion as

1a : the state of a religious <a nun in her 20th year of religion
b (1) : the service and worship of God or the supernatural (2) : commitment or devotion to religious faith or observance
2: a personal set or institutionalized system of religious attitudes, beliefs, and practices
3archaic : scrupulous conformity : conscientiousness
4: a cause, principle, or system of beliefs held to with ardor and faith

and denomination (for our purpose)

4: a religious organization whose congregations are united in their adherence to its beliefs and practices

The definitions above are of the world. Man putting his mark and control on how to approach God, how to worship God, how to believe and partake in something greater than ourselves. Minimizing what God laid out from the very foundations of the beginning.

Look at what God's Word says in James 1:27

27 Pure religion and undefiled before God and the Father is this, To visit the fatherless and widows in their affliction, and to keep himself unspotted from the world. 

This is the Bible's only definition of religion, it seems pretty simple and unlike any definition of the world. Pure religion defines love for the fatherless and widows in their affliction, and purity from the world. This seems to echo the greatest commandment and second from Matthew 22:36-40.

When we accept Christ as our Savior and Lord, we immediately become members of the Church of the Body of Christ. This is the only Church that God's Word mentions.

Romans 12:5

5 So we, being many, are one body in Christ, and every one members one of another.

1 Corinthians 12:12-14 and 27-28

12 For as the body is one, and hath many members, and all the members of that one body, being many, are one body: so also is Christ.
13 For by one Spirit are we all baptized into one body, whether we be Jews or Gentiles, whether we be bond or free; and have been all made to drink into one Spirit.
14 For the body is not one member, but many.

27 Now ye are the body of Christ, and members in particular.
28 And God hath set some in the church, first apostles, secondarily prophets, thirdly teachers, after that miracles, then gifts of healings, helps, governments, diversities of tongues.

Colossians 1:18

18 And he is the head of the body, the church: who is the beginning, the firstborn from the dead; that in all things he might have the preeminence.

This is the Church, the Church of the Body of Christ. We are the Church, each Christian, one who as accepted Jesus Christ as his or her personal Savior and believes that Jesus is the risen Son of God.
There are no denominations, Catholic, Baptist, Methodist, Pentecostal, etc. Religions all fall away. They are man putting himself before God, exerting control and himself on others, foolishly trying to increase what is already perfect and undefiled.

There is only our God and our belief in him.

This is satan's great lie, man against man, brother against brother, believer against believer, to corrupt and defile God's grace and mercy through the hand of man.

When what we should be doing is in Romans 12:10

10 Be kindly affectioned one to another with brotherly love; in honour preferring one another;

As members of the Church of the Body of Christ, as Christians, we should love one another as brothers in Christ, and love our neighbors as we love ourselves, so that they might join the Church of the Body of Christ.