Two things strike me as I read this sentence. One is “great power” and the other is “great grace.” It is possible to testify to the resurrection of the Lord Jesus without great power, and it is possible to have a body of believers who do not have great grace resting upon them.
Something special preceded that statement of grace in Acts: "All the believers were one in heart and mind. No one claimed that any of his possessions was his own, but they shared everything they had" (Acts 4:32).
People were not attached to their possessions, and they were of one heart. Is this why there was great power and great grace? No. This was evidence of that power and grace. Here is the reason for it: "After they prayed, the place where they were meeting was shaken. And they were all filled with the Holy Spirit and spoke the word of God boldly" (Acts 4:31).
The Holy Spirit was the source of the power and grace. The Christians were all filled with the Spirit because they all prayed.
Are you of one heart with the Christians around you? Are you attached to your possessions? Do you speak the word of God with boldness? Do you give your testimony of the resurrection of the Lord Jesus with great power? Maybe you are not filled with the Holy Spirit.