They were all filled with the Holy Spirit and continued to speak the word of God with boldness.  Acts 4:31

Being Spirit-filled means to be under the influence of—under the control of—in submission to the Holy Spirit.  And though we are told that there are many fruits that come as a result of being filled with the Holy Spirit, we learn in Acts 2 and in Acts 4 that Spirit-filled people speak the word of God with boldness. 

God calls for His people to boldly proclaim the truth of His message and then trust in the Spirit of God to use us, the people of God preaching and teaching the Word of God, to bring people to saving faith so that His Kingdom advances.  A good way to test yourself to see if you are filled with His Holy Spirit is by asking yourself whether or not you are being faithful to God by sharing His message when the opportunity presents itself.    

Peter and John and others in the early church most certainly did.  We learn in the first part of Acts that they were faithful to share God’s message and when opposition came they did not run and hide—they did not shrink back, but they expected it and were submissive to it and were spirit-filled and took every opportunity to proclaim God’s message of redemption with boldness.