What does it mean that the Macedonians first gave themselves to the Lord?
It means the Macedonians entered God's saving grace and surrendered to Him before any offering. Their giving followed their commitment to the Lord and their openness to Paul's ministry. This was grace-motivated generosity, not compliance with a set of rules and regulations.
Les explains that New Testament giving is not legally bound or governed by rules and regulations; the saints give as God leads, as He has prospered them, and as the Lord lays it on their heart. The Macedonians’ example is that they first gave themselves to the Lord—that is, they stepped into God’s saving grace—and then they opened themselves to the Apostle Paul and his ministry. Only after that surrender did their financial participation follow. Paul then desired Titus, who had begun the work, to finish in the Corinthians the same grace, picking up the mantle from Paul to gather offerings for the poor saints in person.
And this they did, not as we hoped, but first gave their own selves to the Lord, and unto us by the will of God.