The "word of His grace" in this context is most specifically the whole gospel. Notice especially that Paul says he is committing us to the word of His grace which is able to build us up, to edify us. We could think of it edifying us in terms of building a muscle or of erecting a structure. To put it another way, the word of grace matures us. We usually think of maturity, which is a building of personality and character, as a growth from childhood and all of its weaknesses to a stable adult. Another way of putting it would be, "The word of grace enables us to go on to perfection."
What is beginning to open up here is something very beautiful. Grace does not end when God forgives us. The grace of God continues to add to what was originally given, because if He stopped giving things with the forgiveness of sin, that would be the end of growth; it would stop right there. Forgiveness is only the beginning portion of a process, for God keeps giving us grace to enable us to mature, to grow in grace and knowledge (II Peter 3:18), to grow to the measure of the stature of the fullness of Christ (Ephesians 4:13). We would never get there unless God continued to pour out His grace on us.
The apostle Paul shows grace to be something that is dynamic and active. Remember Jesus said, "The words that I speak to you [the word of grace] are spirit and they are life." There is power in them!
What is the gospel? It is words. It is good news, but it is composed of words of power. This word confers a blessing that is unique, that enables us to mature spiritually. Words—any words—have the power to build or to destroy, to encourage or discourage. They can either be true or they can be lies. They can inspire or they can sadden and depress. It all depends on how they are used, the attitude in which they are used, and how they are arranged.
The gospel is an arrangement of true words that fill us with purpose for living and show us how that purpose can be obtained. It comes completely as a gift; we are favored. The word of grace brings delight and salvation—an arrangement of words given in a loving attitude by a loving God.
John W. Ritenbaugh
Grace Upon Grace