1:12-14 Though, as a sinner, the apostle could only rejoice and glory in Christ Jesus, yet, as a believer, he might rejoice and glory in being really what he professed. Conscience witnesses concerning the steady course and tenor of the life. Thereby we may judge ourselves, and not by this or by that single act. Our conversation will be well ordered, when we live and act under such a gracious principle in the heart. Having this, we may leave our characters in the Lord's hands, but using proper means to clear them, when the credit of the gospel, or our usefulness, calls for it.
14. in part—In contrast to "even to the end": the testimony of his life was not yet completed [Theophylact and Bengel]. Rather, "in part," that is, some of you, not all [Grotius, Alford]. So in 2Co 2:5; Ro 11:25. The majority at Corinth had shown a willing compliance with Paul's directions in the first Epistle: but some were still refractory. Hence arises the difference of tone in different parts of this Epistle. See Introduction.
your rejoicing—your subject of glorying or boast. "Are" (not merely shall be) implies the present recognition of one another as a subject of mutual glorying: that glorying being about to be realized in its fulness "in the day (of the coming) of the Lord Jesus."