By giving themselves to God first and then to their fellow believers, the Macedonians demonstrated their sincere desire to be made perfect. Paul quoted their example in the hope of persuading the Corinthians to do likewise.
The secret of the Macedonians’ zeal was that first, they gave themselves to the Lord. Everything else flowed from that. As a result, when they showed mercy they were not filled with pride but rather displayed great humility and heavenly wisdom.
"And" this, "not as we hoped." This he says with reference both to the amount and to their afflictions. 'For we could never have hoped,' he says, 'that while in so great affliction and poverty, they would even have urged us and so greatly intreated us.' He showed also their carefulness of life in other respects, by saying,
"But first they gave their own selves to the Lord, and to us by the will of God."
'For in everything their obedience was beyond our expectations; nor because they showed mercy did they neglect the other virtues,' "but first gave themselves to the Lord." What is, "gave themselves to the Lord?" 'They offered up [themselves]; they showed themselves approved in faith; they displayed much fortitude in their trials, order, goodness, love, in all things both readiness and zeal.' What means, "and to us?" 'They were tractable to the rein, loved, obeyed us; both fulfilling the laws of God and bound unto us by love.' And observe how here also he again shows their earnestness , saying, "gave themselves to the Lord." They did not in some things obey God, and in some the world; but in all things Him; and gave themselves wholly unto God. For neither because they showed mercy were they filled up with senseless pride, but displaying much lowlymindedness, much obedience, much reverence, much heavenly wisdom, they so wrought their almsdeeds also. But what is, "by the will of God?" Since he had said, they "gave themselves to us," yet was it not "to us," after the manner of men, but they did this also according to the mind of God.
[AD 384] Ambrosiaster on 2 Corinthians 8:5