"All authority in heaven and on earth have been given to me. Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to obey all that I have commanded you. And behold, I am with you always, to the end of the age." (Matthew 28:18-20)
The basis of the call to to go to all the nations and make disciples is the authority of Jesus. His authority, though, comes from his sacrifice—his suffering and death. Because he suffered well, God rewarded him with authority and power. That authority is the basis of our work. Essentially it's because Jesus said so.
Thus, we should have all the confidence in the world that the one who calls us to act in his name will cause us to prevail. In the long run, the one who triumphed over death will not be defeated by evil and neither will we. "The gates of hell shall not prevail." While we may not have the power within ourselves to triumph, we can have the confidence to rely on the one who does have such power within himself.
So when you pray, pray boldly, like one who has at your back the Lord of heaven and earth, to whom has been given all authority and power. He is with you, even to the end of the age.
Who are you praying for today?