Every christian has been called by God to be a missionary. We are His Ambassadors.
2 Corinthians 5:20 Now then we are ambassadors for Christ, as though God did beseech you by us: we pray you in Christ's stead, be ye reconciled to God.
It is not just those that have surrendered to foreign missions. God has called every Christian to share the love of Jesus and the good news of the cross of Christ with others in the place He has put each individual. Your school, your workplace, or your retirement village is your mission field.
The question of the day is, however, "What is our motivation?" Do we simply witness because we feel guilty if we don't? Do we tell others of Jesus just because we feel sorry for them that they may wind up spending eternity without God in a place called Hell? These may be reasons, but they are not sufficient motivation to keep us on the job as ambassadors for Him.
The Bible gives us a deeper, more profound motivation. That is, God's love for us. The more we understand His love for us, the more we will respond to that love in telling others about it. Take a look at 2 Corinthians 5:14-15, "For the love of Christ constraineth us; because we thus judge, that if one died for all, then were all dead: And that he died for all, that they which live should not henceforth live unto themselves, but unto him which died for them, and rose again."
The father does not really care much about his daughter's doll. But if that doll goes missing, he will search until it is found. This is not because he loves the doll. It is because he loves the child. We do not love other people enough for them to be our motivation. We search for the lost and tell them of salvation because God loves them.
Living a missional life (missionary life) is a response of the heart to an understanding of the love of God.