What causes the Christians to be so bold? It is not themselves but the righteousness they have in the Lord Jesus Christ. Apart from Christ, Christians have no power against sin on their own nor do they have that power to overcome their fears. That was an important lesson Peter learned when he denied the Lord Jesus Christ thrice during the night of the arrest. He was afraid apart from the grace of God but after he learned to stand by God's grace, he was given that boldness to proclaim to Gospel of Jesus Christ.
Christians would forsake all rather than Christ and to offend all rather than offend Him is because of this new nature in them. Titus 2:11-14 says that the grace of God leads from sinfulness to living righteously and soberly in this fallen world. They might be afraid for now but God always finds a way to put Christians in prayer, make them pray for that boldness to stand up for Him.