What Did Jesus Mean By "Church"?

The first time the word church appears in the Bible, it’s uttered by Jesus. Did His disciples know what He meant? If so, how?

Matthew 16:13-19 (NIV)

13 When Jesus came to the region of Caesarea Philippi, he asked his disciples, “Who do people say the Son of Man is?”

14 They replied, “Some say John the Baptist; others say Elijah; and still others, Jeremiah or one of the prophets.”

15 “But what about you?” he asked. “Who do you say I am?”

16 Simon Peter answered, “You are the Messiah, the Son of the living God.”

17 Jesus replied, “Blessed are you, Simon son of Jonah, for this was not revealed to you by flesh and blood, but by my Father in heaven. 18 And I tell you that you are Peter, and on this rock I will build my church, and the gates of Hades will not overcome it. 19 I will give you the keys of the kingdom of heaven; whatever you bind on earth will be bound in heaven, and whatever you loose on earth will be loosed in heaven.”

The Greek word for church here is ekklesia. It means an "assembly", and was often used at the time to describe a gathering of citizens or government officials in a public space, as in Acts 19:39. It is highly likely that Jesus was speaking Aramaic or Hebrew and not the Greek of the gospel manuscripts that we have today. The Septuagint, the Greek version of the Old Testament, uses the same word to describe the assembly of the tribes of Israel. 

Ekklesia is a combination of the prefix ek or ex, which means “out of ” and the root kaleo, which means "to call". Literally the word means, “the called out ones”. Mark 3:13 - And he went up on the mountain and called to him those whom he desired, and they came to him. The disciples had experienced being called out from their communities, jobs, and families to follow Jesus. With their Jewish heritage, they likely would not divorce the word from its Old Testament meaning: a gathering of believers for a spiritual purpose, to hear God’s Word or to accomplish His direction.