While God has delegated angels to make special pronouncements for Him, He has not given them the privilege of proclaiming the gospel message today. Why this is so, Scripture does not say. Perhaps spirit-beings who have never experienced the effects of sin would be unable to preach with understanding.
Rather, God has commanded the church to preach. This great task is reserved for believers. Only people can speak salvation’s experience to other people. God has, however, assigned angels to assist those who preach. Their assistance includes the use of miraculous and corroborating signs. Missionaries have reported many wonderful incidents where angels seemed to help them as they proclaimed the gospel. My wife, whose parents were missionaries to China, can remember many instances in her life where angels must have intervened in the ministry of her father and his fellow missionaries.
At any rate, you and I have the privilege of conveying a message to people from God in Heaven, a message that angels cannot speak. Think of that! No angel can be an evangelist. No angel can pastor a church. No angel can do counseling. No angel can enjoy sonship in Jesus or be partaker of the divine nature or become a joint heir with Jesus in His kingdom. You and I are a unique and royal priesthood in the universe, and we have privileges that even angels cannot experience.
—Adapted from Angels by Billy Graham