Mission of the modern church and the mission of the prophets

What is the purpose of God within the missionary activity of the contemporary church as outlined in the Old Testament?

Based on scripture, God's purpose initiated with the prophets' message to spread the Word of God to everyone in the nation. As Duguid (2012) notes, "In the beginning, God created Adam and Eve to be a worshipping community: He would be their God and they would be His people". Beginning with the Book of Deuteronomy, God establishes a holy people unto himself with the people of Israel (Deut. 28:9, KJV). Despite His disappointment with Israel's acts that caused them to rebel against God and become a sinful nation, God promised the nation of Israel a sign by which the people would once again be renewed [as a holy people under the name of Immanuel [Jesus](Isaiah 7: 14, KJV).
Jesus would be a sign for the nations, who would" assemble the outcasts of Israel, and gather together, "the dispersed of Judah from the four corners of the earth (Isa. 11: 12).Based on the Christian faith, Jesus is a seed of Abraham. The Bible says, "And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise (Gal 2:29). God's plan for Abraham was to bestow him with earthly blessings, which in turn would become a blessing to other nations. The contemporary church has the same mission-- represent God's blessing in Jesus Christ, and share those blessings with others. In the same way that Israel was to be a holy nation that assumed a relationship with God, and towards other nations; so is today's church. However, we are set apart in the same way that Israel was set apart-- as a holy people, to be a seed, bless the people and be a light on this earth.
Somehow Israel either forgot, or lost interest in its position, and ignored this calling and sharing mission. Similarly, the church forgets its mission is to share the gospel of Jesus Christ with the rest of the world. Instead, sometimes Christians becomes less helpful, interested in their own piety and more hypocritical of others, who may look, think and worship differently than they do. It is to be noted that similar actions took place among Israelites, who had become "laden with iniquity, a seed of corruption and evildoers", and provoked God of Israel to anger" (Isa. 1: 4). However, Isaiah reminds us that, this servant "Israel" took flesh in the person of Jesus Christ. With His coming, Christ reenacted Israel's history (from Egypt) so that He could be the true son whom God called out of Egypt (Matt. 2:15).
Thus, Jesus fulfilled God's original design for human holiness through the New Israel prophesized by Isaiah. According to Duguid (2012), Jesus saw himself as the new Israel "described by the same terminology that described Israel in the Old Testament". Indeed, we are a holy people, a chosen nation through the redemptive blood of Jesus Christ (by His death on the cross), who saved humans from being lost to sin. This is a message that Jesus taught that fulfills God's purpose. It is the message he mandated His disciples to teach to all nations (Acts. 1: 8, KJV). This is the mission of the contemporary church to continue to communicate the gospel of Jesus Christ, and tell the world of His saving grace.
Reference:
Duguid (2012). The church and Israel in the Old Testament. Retrieved from, http://www.ligonier.org/learn/articles/the-church-and-israel-in-the-old-testament/.