That this is the meaning of the testimony, that is, of the words, "that a word may be given to me when my mouth is opened," is proved by what follows. He says, "and confidently to make known the mystery of the gospel," and not in parables and examples, as the prophets spoke, and as the Lord Himself spoke while He was still in the body: "The hour is coming when I shall no longer speak to you in parables, but shall tell you plainly of the Father" (Jn 16:25).
Only the one whose heart does not trouble him can speak such a confident word. "For if our heart does not restrain us, we have confidence toward God, and we shall receive from Him whatever we ask" (1 Jn 3:20). Therefore one is rarely found who will confidently make known the mystery of the gospel, because one is rarely found who has confidence in God. For "Who will boast that he has a blameless heart, or who will say that he is clean from sins?" (Prov 20:9).
After this it is necessary to explain briefly what is meant by, "for which I am an ambassador in chains," namely, in order to make known the mystery of the gospel.
Indeed, whoever understands this simply will say that he sent the present epistle from confinement in prison and in chains in Rome. Another will argue that he spoke this way because of the lowliness of the body and these chains that surround us. For we do not yet know as we ought to know, but see obscurely, as in a mirror.
In truth, the Apostle can reveal the mysteries of the gospel with confidence only when he casts off the chains and goes out free from prison. Yet one whose citizenship is in heaven should perhaps not be considered without chains, even while he is in chains. Of such a man it may be said: "You are not in the flesh but in the Spirit, if indeed the Spirit of God dwells in you" (Rom 8:9).