Paul had said that God exhorted by his means. In chapter 6 the affection of the apostle carries on by the Spirit this divine work, beseeching the Corinthians that it might not be in vain in their case that this grace had been brought to them. For it was the acceptable time, the day of salvation.  The apostle had spoken of the great principles of his ministry, and of its origin. He reminds the Corinthians of the way in which he had exercised it in the varied circumstances through which he had been led. The cardinal point of his service is that he was the minister of God, that he represented Him in his service. This rendered two things needful: first, that he should be in all things without reproach; and then that he should maintain this character of God's minister, and the exercise of his ministry, through all the opposition, and in all the circumstances through which the enmity of man's heart, and the cunning even of Satan, could make him pass. Everywhere and in all things he avoided, by his conduct, all real occasion of being reproached, in order that no one should have room to blame the ministry. He approved himself in all things as a minister of God, worthily representing Him in whose name he spoke to men; and that with a patience, and in the midst of persecution and contradiction of sinners, which shewed an inward energy, a sense of obligation to God, and a dependence on Him, which the realisation of His presence and of our duty to Him can alone maintain. It was a quality which reigned through all the circumstances of which the apostle speaks, and had dominion over them.
Thus he shewed himself to be the minister of God in everything which could test him; in pureness, in kindness, in love; as a vessel of power; whether disgraced or applauded; unknown to the world, and known and eminent; outwardly trodden under foot of man and chastened, inwardly victorious and joyful, enriching others, and in possession of all things. Here ends his description of the sources, the character, the victory over circumstances, of a ministry which displayed the power of God in a vessel of weakness, whose best portion was death.
The restoration of the Corinthians to a moral state befitting the gospel, associated with the circumstances through which he had just been passing, had allowed him to open his heart to them. Pre-occupied till now with his subject of the glorious Christ, who, having accomplished redemption, sent him as the messenger of the grace to which that redemption had given free course, and having spoken with a free heart of all that was comprised in his ministry, he returns with affection to his beloved Corinthians, shewing that it was with them that he had all this openness, this enlargement of heart. "My mouth is open unto you, O Corinthians," he says, "my heart is enlarged; ye are not straitened in me, but in your own affections." As a recompense for the affections that overflowed from his heart towards them, he only asks for the enlargement of their own hearts.
He spoke as to his children. But he avails himself of this tender relationship to exhort the Corinthians to maintain the place in which God had set them: "Be not in the same yoke with unbelievers." Having a hold upon their affections, and rejoicing deeply before God in the grace which had restored them to right sentiments, his heart is free to give way, as though beside himself, to the joy that belonged to him in Christ glorified: and, with a sober mind after all when his dear children in the faith were in question,  he seeks to detach them from all that recognised the flesh, or implied that a relationship which recognised it were possible for a Christian-from everything that denied the position of a man who has his life and his interests in the new creation, of which Christ is the Head in glory. An angel can serve God in this world: little would it concern him in what way, provided that way was God's; but to associate himself with its interests, as forming a part of it, to ally himself with those who are governed by the motives that influence the men of this world, so that a common conduct would shew that the one and the other acted according to the principles that form its character, would be, to those heavenly beings, to lose their position and their character. The Christian, whose portion is the glory of Christ-who has his world, his life, his true associations, there where Christ has entered in-should not either; nor can he, as a Christian, put himself under the same yoke with those who can have only worldly motives, to draw the chariot of life in a path common to both.
What communion is there between Christ and Belial; between light and darkness; faith and unbelief; the temple of God and idols? Christians are the temple of the living God who dwells and walks among them. He is a God to them; they are a people to Him. Therefore must they come out from all fellowship with the worldly, and be separate from them. As Christians, they must stand apart, for they are the temple of God. God dwells among them and walks there, and He is their God. They are therefore to come out from the world and be separate, and God will own them, and will be to them in relationship of a Father with sons and daughters who are dear to Him.
This, observe, is the special relationship which God assumes with us. The two preceding revelations of God with men are named here, and He takes a third. To Abraham He revealed Himself as Almighty; to Israel as Jehovah or Lord. Here the Lord Almighty declares that He will be a Father to His own, to His sons and daughters. We come out from among the worldly, for it is just that (not physically out of the world, but while in it), in order to enter into the relationship of sons and daughters to the Almighty God: otherwise we cannot practically realise this relationship. God will not have worldlings in relation with Himself as sons and daughters; they have not entered into this position with regard to Him. Nor will He recognise those who remain identified with the world, as having this position; for the world has rejected His Son, and the friendship of the world is enmity against God: and he who is the friend of the world is the enemy of God. It is not being His child in a practical sense. God says therefore, "Come out from among them, and be separate, and ye shall be to me for sons and daughters." Remember that it is not a question of coming out of the world-it is while we are in it-but of coming out from among the worldly, to enter into the relationship of sons and daughters, in order to be to Him for sons and daughters, to be owned of Him in this relationship. 
But it is not only that from which we are separated to be in this position of sons and daughters that engages the apostle's attention, but the legitimate consequences of such promises. Sons and daughters of the Lord God Almighty, holiness becomes us. It is not only that we are to be separate from the world; but, in relationship with God, to cleanse ourselves from all filthiness of the flesh and spirit: holiness in the outward walk, and that which is quite as important with regard to our relationship to God, purity of thought. For, although man does not see the thoughts, the flow of the Spirit is stopped in the heart. There is not enlargement of heart in communion with God, It is much if His presence is felt, His relationship to us realised; grace is known, but God scarcely at all, in the way in which He makes Himself gradually known in communion.