What is the meaning of Acts 23:16-21?

16 And when Paul’s sister’s son heard of their lying in wait, he went and entered into the castle, and told Paul. 17 Then Paul called one of the centurions unto him, and said, Bring this young man unto the chief captain: for he hath a certain thing to tell him. 18 So he took him, and brought him to the chief captain, and said, Paul the prisoner called me unto him, and prayed me to bring this young man unto thee, who hath something to say unto thee. 19 Then the chief captain took him by the hand, and went with him aside privately, and asked him, What is that thou hast to tell me? 20 And he said, The Jews have agreed to desire thee that thou wouldest bring down Paul to morrow into the council, as though they would enquire somewhat of him more perfectly. 21 But do not thou yield unto them: for there lie in wait for him of them more than forty men, which have bound themselves with an oath, that they will neither eat nor drink till they have killed him: and now are they ready, looking for a promise from thee. (Acts 23:16-21 KJV)

William Burkitt’s Commentary

Note here, 1. That no conspiracies are or can be kept secret from God who can both detect them and defeat them at his pleasure.
Note, The remarkable providence of God in bringing this conspiracy to the knowledge of St. Paul’s sister’s son: he was perhaps near, when the conspirators were contriving the mischief, and overheard them. It is happy for the innocent that the malicious cannot keep their own counsel. God oftentimes causes the tongues of his people’s enemies to fall upon themselves, and they discover the wicked purposes of their hearts, which none but themselves were privy to.
Note, 3. How the hearts of all men are in the hand of the Lord, and how he turns them as he pleaseth. This is evident from the chief captain’s great humanity towards St. Paul, and his courteous humility towards his sister’s son, taking the youth by the hand and as readily giving him both audience and dispatch; God has such sovereignty and dominion over the hearts of men, that he can instantly incline them as he pleases, and make even enemies become benefactors at his pleasure.