Joshua 1:9 (ESV) … “Have I not commanded you? Be strong and courageous. Do not be frightened, and do not be dismayed, for the Lord your God is with you wherever you go.”
In the years to come, whenever Joshua faced an enemy and was tempted to be afraid, he would remember that he was a man with a divine commission—and his fears would vanish. Whenever things went wrong, and he was tempted to be dismayed, he would recall God’s command—and take new courage. Like Moses before him, and Samuel and David after him, Joshua had a divine mandate to serve the Lord and do His will—and that mandate was sufficient to carry him through.
Likewise, we have been given a divine mandate to go and be a servant witness of the Lord. That mandate is given by Jesus, and we call it the “Great Commission,” Matthew 28:16-20.
The same promise that God gave to Joshua He gives to us. We too, can be strong and courageous. We also, need not be dismayed by the things that Satan and this world might throw at us. Wherever we go, the Lord is with us. He has promised to never leave us. He has promised us the indwelling of the Holy Spirit. Therefore, we must never be afraid to serve our Lord but trust in Him fully and completely.
 Wiersbe, W. W. (1996). Be Strong (p. 28). Wheaton, IL: Victor Books.