"The righteous are bold as a lion." (Proverbs 28:1)
A holy boldness is imparted to those who seek to speak the truth of God (Acts 4:31). The miracle of the Pentecost outpouring of the Holy Spirit was followed by several incidents where the various apostles and early Christian leaders spoke "boldly in the Lord" (Acts 14:3, etc). Where does this boldness come from?
The Presence of the Holy Spirit: The Sanhedrin "saw the boldness of Peter and John" when they were dragged before them (Acts 4:13), after they had healed the lame man shortly after Pentecost. Peter was "filled with the Holy Ghost" (Acts 4:8) and boldly answered the farcical questioning of those self-righteous leaders, and they "took knowledge of them, that they had been with Jesus" (Acts 4:13). When we speak with God's authority, we speak boldly.
The Words of God's Word: The first church prayed "that with all boldness they may speak thy word" and they were enabled to speak "the word of God with boldness" (Acts 4:29, 31). When Paul was starting the church in Ephesus, he "spake boldly for the space of three months, disputing and persuading the things concerning the kingdom of God" (Acts 19:8). We should have boldness when we have opportunity to "make known the mystery of the gospel" (Ephesians 6:19).
The Assurance of a Righteous Life: "According to my earnest expectation and my hope, that in nothing I shall be ashamed, but that with all boldness, as always, so now also Christ shall be magnified in my body, whether it be by life, or by death" (Philippians 1:20). Several godly traits of righteous men are given in Hebrews, "so that we may boldly say, The Lord is my helper, and I will not fear what man shall do unto me" (Hebrews 13:6).
These are from where that boldness comes. HMM III