Please, why is the church called the body of Christ and at the same time called the bride of Christ?
People are known by a variety of names and titles because one name doesn't completely describe a person or his relationship with other people. I'm known as "Jeff" to friends, but in business dealings, I usually get called "Jeffrey" or "Mr. Hamilton." The young people call me "Mr. Jeff" as a compromise between respectfully formal and to say they see me as a friend. But my college students call me "professor." My wife may call me "Honey," but my children call me "Dad."
The church is referred to by a variety of names because one name just doesn't capture what the church is.
The name "church" is used to translate the Greek word ekklesia. A literal translation means "the called out" and refers to an assembly. It is used to refer to all Christians who are called out of the world of sin (Hebrews 12:23) or it can refer to Christians in a region who are called out to worship together (I Corinthians 1:2).
But "called out" doesn't really tell us why we are called; at least, not directly. After all, a rioting mob is also a "called out" gathering. Hence, we also refer to the church as the kingdom. This recognizes that Jesus is Lord of His people and each Christian is a citizen in his kingdom (Colossians 1:13). It emphasizes that we are a people of laws and that Christ is our lawgiver (Luke 6:46).
In a typical kingdom, you might not have direct interaction with the ruler or even most of your fellow citizens. This doesn't capture the idea of the church well. Christians have a close relationship with God. He is our Father. This makes each Christian a member of God's family (Ephesians 2:19; I Timothy 3:15). This is why we refer to each other as brother or sister. It is also why the love for each other defines Christians (John 13:35).
Not every family is at peace. Too often members squabble with each other. The church is referred to as the body of Christ to emphasize the unity of its members. Unity is defined by the head who is Jesus. Just as a body is composed of different organs, each Christian performs different roles that together make the church function. And all members of the church are needed (I Corinthians 12:12-27).
In both a household and a member of a body, the members typically don't get to chose to be where they are, so in this aspect, these names for the church don't quite capture the nature of the church. Thus, the church is called the bride of Christ. Jesus is seen as the groom. The actual "marriage" will take place when we reach heaven (Revelation 21:2). But currently, the church is committed to Jesus and is to be free from sin (pure) (Ephesians 5:22-33). This again emphasizes that Jesus is the head of the church but our relationship to Jesus is seen as a voluntary submission to him.
This name puts greater emphasis on the fact that Christians are set apart for a holy purpose. The church is seen as a spiritual temple -- a holy place to worship God. The cornerstone is Jesus. The apostles and prophets are the foundation build against the cornerstone. Each Christian become a piece of the structure built on the foundation. Once again, Jesus sets the rules and the church acts as a unified whole that has a purpose in its existence.
Each name helps us better understand what the church is, but each doesn't quite cover what the whole of the church is. The names are just different ways to look at the church.