For which I am an ambassador in bonds: that therein I may speak boldly, as I ought to speak.
Paul considered himself and ambassador (representative) of the mystery of the Gospel (considering Ephesians 3, particularly the mystery of the Gospel to the Gentiles). This is the second time Paul mentions boldness.
Examples of boldness:
"Then Peter, filled with the Holy Ghost, said unto them, Ye rulers of the people, and elders of Israel, If we this day be examined of the good deed done to the impotent man, by what means he is made whole; Be it known unto you all, and to all the people of Israel, that by the name of Jesus Christ of Nazareth, whom ye crucified, whom God raised from the dead, even by him doth this man stand here before you whole. This is the stone which was set at nought of you builders, which is become the head of the corner. Neither is there salvation in any other: for there is none other name under heaven given among men, whereby we must be saved. Now when they saw the boldness of Peter and John, and perceived that they were unlearned and ignorant men, they marvelled; and they took knowledge of them, that they had been with Jesus." (Acts 4:8-13)
"And now, Lord, behold their threatenings: and grant unto thy servants, that with all boldness they may speak thy word, By stretching forth thine hand to heal; and that signs and wonders may be done by the name of thy holy child Jesus." (Acts 4:29-30)
"And when they had prayed, the place was shaken where they were assembled together; and they were all filled with the Holy Ghost, and they spake the word of God with boldness." (Acts 4:31)
"According to the eternal purpose which he purposed in Christ Jesus our Lord: In whom we have boldness and access with confidence by the faith of him." (Ephesians 3:11-12)
Boldness comes by the filling of the Holy Ghost. Boldness is granted by prayer. One cannot decide, "I'm going to be bold for Christ." Rather, it comes by following as Christ leads, granted as an answer to prayer for boldness. Do I pray for boldness? (for myself and others) Do I pray that God would help me (and others) to speak as He would have us to speak?