One of our brothers Br. Steve, ST was able to find a letter, from our founder Fr. Thomas Judge, in which we think the saying originated.
... a power for good
Our founder Fr. Thomas A. Judge, CM said,” throw yourself confidingly, generously into the hands of Divine Providence and never worry about tomorrow. You will always find then, that your program is God’s plan. Leave home with only this thought, “I wish to be good; I wish to do good; I wish to become a power for good.”
Although our sisters are trained in many professions and active in many types of ministries what is important is not the type of ministry we do but how we do it. The way we are in prayer, in ministry and with others needs to reflect our personal love of God our Father, of his son Jesus and of the Holy Spirit who abides with us.
"Our specific mission is the preservation of the faith in those areas and among those people who are spiritually neglected and abandoned, especially the poor. Our chief effort is to develop a missionary spirit in the laity, with the goal that every Catholic be an apostle." MSBT Rule of Life #5