What happens next catches us off guard. Not only is Timothy not the bearer of the present letter, but he is also well known to the Philippians (see comment on 1:1); yet Paul proceeds to "commend" him. Moreover, and significantly, the commendation does not speak first about their own long-term knowledge of him but about Paul's having no one else like him (literally, "of like soul" like-minded). This sounds so much like the appeal in verses 2-4, one must assume it to be intentional—and for their sakes.
But "like-souled" to whom? Although the phrase is ambiguous as it stands, most likely Paul means "like-minded to me," rather than no one else's being like-minded to Timothy. Thus with a slight wordplay on being cheered (literally "good souled"), Paul emphasizes that the primary reason for Timothy's coming is that he carries Paul's own deep concerns at heart. When he arrives, therefore, they can count on it: "he will genuinely show concern for your affairs" (repeating from v. 19), especially with regard to their standing firm in the one Spirit in the face of opposition in Philippi (1:27-30). Paul thus begins with the reasons from his perspective that only Timothy will do; he will go on in 2:22 to the reason from their perspective. But before that he takes a broadside at some people who do not have Timothy's (thus his own) mindset.
With an explanatory for, Paul contrasts Timothy's like-minded concern for the Philippians' welfare with the mindset of others who are looking out for their own interests, not those of Christ Jesus. That they stand in direct contrast to Timothy, with language reminiscent of verse 4, indicates that "looking out for the interests of Christ Jesus" equals "looking also to the interests of others." This is what the preceding section was all about, Paul's own seeking the interests of Christ Jesus, by appealing to the Philippians to work out—among themselves and in Philippi—the salvation that Christ has brought them.
But we are poorly prepared for this sentence. Verses 20 (Paul's reasons from his perspective for sending Timothy) and 22 (Paul's reasons from their perspective) make perfectly good sense. So why this interruption, and who are these people, especially since Paul's brush sweeps so widely: everyone is like this? We begin with the everyone (or "all"), which can only refer to people in Rome, not Philippi, and probably means something like "the whole lot of them." But "the whole lot of whom," since he hardly intends to indict every believer in Rome? The form of the contrast, including I have no one "like-minded" in verse 20, at first blush sounds as if he intended to contrast Timothy with other coworkers who might be available for such duty. But Paul does not say that; and the content of the sentence makes such a view even difficult to imagine. Given what we know of Paul elsewhere and the high regard with which he holds those who travel with him, and that in 4:21 he sends greetings from "the brothers who are with me," it seems highly unlikely that he should here slander them with this kind of barrage. And in any case, in light of the next verse it also seems unlikely that he would have even considered sending anyone else.
The contrast, therefore, is not between Timothy and other coworkers who could make this trip but are too self-serving to send, but between Timothy's character qualifications and those of some other people who came to mind as Paul was dictating. These people are condemned precisely because they lack the two essential qualities noted of Timothy in verse 20: (1) like-mindedness with Paul, which expresses itself in (2) genuine concern for others and thus exemplifies the character of the gospel that was presented in 2:3-4. Probably, then, this aside looks in two directions at once. What prompts it are people like those already mentioned in 1:15 and 17, who preach Christ but "not purely/sincerely," and who therefore are not truly doing so for Christ's sake. But as verse 20 has already hinted, Timothy is being set forth as yet another model of one who "thinks like Christ" and is therefore being singled out for the benefit of some in Philippi who are otherwise-minded (2:3-4). That it is intended in part for Philippi seems verified by Paul's language, which is the clue to much. Such people, he says, "seek" (NIV look out for) their own interests; Paul has already appealed to the Philippians to do nothing out of self-interest but rather in humility to regard the needs of others as having precedence over their own (2:3-4).
From his denunciation of the self-seeking, Paul returns to the commendation of Timothy, reminding the Philippians of their own knowledge of him: you know that Timothy has proved himself. The word "proven character" (NIV proved himself) has been coined from the verb "to put to the test." Because of long associations with Timothy, they know his worth, that his character has been put to the test and thus he has proved himself.
But as with other matters in this letter, Paul's interest in Timothy's "proven character" is not with his character in general but in particular by the way he has served with me in the work of the gospel. As the Philippians well know, that relationship is like that of a son with his father, the apprentice son who exhibits the mind and concerns of his father, alongside whom he has served for so many years. The reminder is similar to that in 1 Corinthians 4:17, "like father, like son": You can count on Timothy's being among you as a son who looks and acts just like me. He has served with me in the work of the gospel sounds very much like what Paul had earlier thanked God for with regard to the Philippians (in 1:5). Thus, like the content of the two preceding verses, this is expressed in terms that recall earlier moments in the letter and therefore is very likely intended also to reinforce the paradigm.
Would that all of us were like Timothy, putting the interests of others as the matter of first importance! Here again the way of humility, taking the lower road by way of the cross, is on full display; and here alone, as the gospel affects the people of God in this way at the core of our beings, can we expect truly to count for the gospel in a world that lives the opposite, not only as a matter of course but for the most part as its primary value. One must "look out for number one," after all. Agreed, as long as the cross dictates that "number one" is one's neighbor and not oneself.