Wherefore, while I have great confidence in Christ to command thee. That is, "though I have authority so that I might justly command thee, yet thy love makes me prefer to entreat thee."

Wherefore, though I might be much bold in Christ - Though I might have much boldness as an apostle of Christ. He means that he was invested with authority by the Lord Jesus, and would have a right, as an apostle, to enjoin what ought to be done in the case which he is about to lay before him; compare 1-Thessalonians 2:6-7.
To enjoin thee that which is convenient - To command what is proper to be done. The word "convenient" here (τὸ ἀνῆκω to anēkō), means that which would be fit or proper in the case; compare the notes at Ephesians 5:4. The apostle implies here that what he was about to ask, was proper to be done in the circumstances, but he does not put it on that ground, but rather asks it as a personal layout. It is usually not best to command a thing to be done if we can as well secure it by asking it as a favor; compare Daniel 1:8, Daniel 1:11-12.

Wherefore, though I might be much bold - It would be better to read: Wherefore, although I have much authority through Christ, to command thee to do what is proper; yet, on account of my love to thee, I entreat thee.
The tenderness and delicacy of this epistle, says Dr. Paley, have long been admired: "Though I might be much bold in Christ to enjoin thee that which is convenient; yet, for love's sake, I rather beseech thee, being such a one as Paul the aged, and now also a prisoner of Christ Jesus, I beseech thee for my son Onesimus, whom I have begotten in my bonds."
There is something certainly very melting and persuasive in this and every part of the epistle. Yet, in my opinion, the character of St. Paul prevails in it throughout. The warm, affectionate, authoritative teacher is interceding with an absent friend for a beloved convert. He urges his suit with an earnestness befitting, perhaps, not so much the occasion as the ardour and sensibility of his own mind. Here also, as everywhere, he shows himself conscious of the weight and dignity of his mission; nor does he suffer Philemon, for a moment, to forget it: "I might be much bold in Christ, to enjoin thee that which is convenient." He is careful also to recall, though obliquely, to Philemon's memory, the sacred obligation under which he had laid him, by bringing him to the knowledge of Christ: "I do not say to thee, how thou owest to me even thine own self besides." Without laying aside, therefore, the apostolic character, our author softens the imperative style of his address, by mixing with it every sentiment and consideration that could move the heart of his correspondent. Aged, and in prison, he is content to supplicate and entreat. Onesimus was rendered dear to him by his conversation and his services; the child of his affliction, and "ministering unto him in the bonds of the Gospel." This ought to recommend him, whatever had been his fault, to Philemon's forgiveness: "Receive him as myself, as my own bowels." Every thing, however, should be voluntary. St. Paul was determined that Philemon's compliance should flow from his own bounty; "Without thy mind would I do nothing, that thy benefit should not be as it were of necessity, but willingly;" trusting, nevertheless, to his gratitude and attachment for the performance of all that he requested, and for more: "Having confidence in thy obedience, I wrote unto thee, knowing that thou wilt also do more than I say." St. Paul's discourse at Miletus; his speech before Agrippa; his Epistle to the Romans; that to the Galatians, Galatians 4:11-20; to the Philippians, Philippians 1:29; Philippians 2:2; the second to the Corinthians, 2-Corinthians 6:1-13; and indeed some part or other of almost every epistle, exhibit examples of a similar application to the feelings and affections of the persons whom he addresses. And it is observable that these pathetic effusions, drawn for the most part from his own sufferings and situation, usually precede a command, soften a rebuke, or mitigate the harshness of some disagreeable truth. Horae Paulinae, p. 334.

Wherefore, though I might be much bold in Christ,.... Or use much freedom of speech in the name of Christ, as an ambassador of his, and great authority as his apostle, which was given him for edification:
to enjoin thee that which is convenient; which became him as a believer in Christ, and a minister of the Gospel; which was his duty, and was obligatory upon him, agreeable to the doctrines of Christ; who taught men to love their enemies, to be reconciled to their brethren, that had offended them, especially when they repented; and therefore it was fit and proper that he should receive his servant again, since God had called him by his grace, and given him repentance for his sins: upon this foot the apostle could have commanded him, as he did in other cases, 2-Thessalonians 3:6, but he chose not to address him in an authoritative way, but by way of entreaty, as follows.

It does not lower any one to condescend, and sometimes even to beseech, where, in strictness of right, we might command: the apostle argues from love, rather than authority, in behalf of one converted through his means; and this was Onesimus. In allusion to that name, which signifies "profitable," the apostle allows that in time past he had been unprofitable to Philemon, but hastens to mention the change by which he had become profitable. Unholy persons are unprofitable; they answer not the great end of their being. But what happy changes conversion makes! of evil, good; of unprofitable, useful. Religious servants are treasures in a family. Such will make conscience of their time and trusts, and manage all they can for the best. No prospect of usefulness should lead any to neglect their obligations, or to fail in obedience to superiors. One great evidence of true repentance consists in returning to practise the duties which have been neglected. In his unconverted state, Onesimus had withdrawn, to his master's injury; but now he had seen his sin and repented, he was willing and desirous to return to his duty. Little do men know for what purposes the Lord leaves some to change their situations, or engage in undertakings, perhaps from evil motives. Had not the Lord overruled some of our ungodly projects, we may reflect upon cases, in which our destruction must have been sure.

Wherefore--Because of my love to thee, I prefer to "beseech," rather than "enjoin," or authoritatively command.
I might . . . enjoin--in virtue of the obligation to obedience which Philemon lay under to Paul, as having been converted through his instrumentality.
in Christ--the element in which his boldness has place.

Wherefore. After this introduction Paul states the purpose of the letter.
Though I might be much bold in Christ. As an apostle, and as the one who gave Philemon the gospel, he had the right to command what is befitting.
Yet. Yet he does not come thus with commands, but as beseeching for love's sake.
Paul the aged. That his appeal may be more sure to touch Philemon, he reminds him that Paul is an old, gray-haired, scarred veteran of Christ, who has grown aged in his service, and is now a prisoner suffering for his Lord.
For my son Onesimus. His spiritual son, whom he in his bonds, while a prisoner in chains, had converted. It is possible that Epaphras met Onesimus, his fellow-townsman in Rome, and brought him to Paul.
Which in time past was to thee unprofitable. He may not only have been a restless, discontented servant, but in addition, he ran away.
But now profitable. Such a change has taken place in him. He has served Paul in his bonds well, and will also serve Philemon well.
Whom I have sent again. Not only that he may make amends to thee for his wrong, but that thou mayst be able to treat him as a brother in Christ.
Mine own bowels. Rather, "My very heart" (Revision). I am so much attached to him. To be unkind to him would wound Paul's very heart.
Whom I would have retained. Would gladly have kept him with me to render for thee the service you would be glad to give me while I am in chains, only (14) without thy mind would I do nothing. He wished, if such a service was rendered, it might be with Philemon's free consent.

I might be bold in Christ - Through the authority he hath given me.