Our Mission: Making Disciples of All Nations

In the Christian world, you will hear of members say phrases like, "going on missions" or "supporting missions."  They are most likely be referring to the word meaning a group of people going to a foreign country to preach the Gospel and/or assist with relief.  

Why does the definition have to include the word "foreign"?  

I mean, Jesus said, "Go into all the world and preach the gospel to all creation."  In another Gospel, He says it this way, "Therefore go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit." 

There is no place in there that I see him implying that you have to travel by plane to get there.