20 And when he came to the den, he cried with a lamentable voice unto Daniel: and the king spake and said to Daniel, O Daniel, servant of the living God, is thy God, whom thou servest continually, able to deliver thee from the lions?
[AD 420] Jerome on Daniel 6:20
Verse 20. "And approaching the pit, he called out to Daniel with a tear-choked voice and addressed him." By his tears he showed his inner emotion, and forgetting his royal dignity, the conqueror ran to his captive, the master to his servant.

"'O Daniel, servant of the living God...'" He calls Him the living God in order to distinguish Him from the gods of the Gentiles, who are but effigies of the dead.

"'Dost thou deem that thy God, whom thou ever servest, has been able to deliver thee from the lions?'" It was not that he had any doubts about the power of the God of whom he had previously affirmed, "Thy God, whom thou ever servest, will Himself deliver thee." But he phrased the sentence doubtfully in order that when Daniel made his appearance unharmed, the king's anger at the princes might seem the more justified, in proportion to the incredibility of the event.

[AD 458] Theodoret of Cyrus on Daniel 6:20
Each of these phrases shows the religious spirit of the king, who complimented Daniel on his piety: first, he calls him not his own servant but God’s; next, he calls Daniel’s God “living;” then, in praise of his piety, “whom you serve with constancy,” that is, you were not prevented from worship under pressure of the law. The question “Was he able to rescue you from the lions’ mouth?” means, “Was it his will to render you impervious against the lions?” After all, he would not have referred to God as powerless after calling him “living.”