John Calvin Commentary 2 Corinthians 8:3

"For according to their power, I bear witness, yea and beyond their power, [they gave] of their own accord," — 2 Corinthians 8:3 (ASV)

To their power, and even beyond their power. When he says that they were willing of themselves, he means that they were, of their own accord, so well prepared for the duty that they needed no exhortation. It was a great thing—to strive up to the measure of their ability; and thus, to exert themselves beyond their ability showed a rare and truly admirable excellence.

Now he speaks according to the common human custom, for the common rule of doing good is what Solomon prescribes (Proverbs 5:15)—to drink water out of our own fountains, and let the rivulets go past, that they may flow onwards to others.

The Macedonians, on the other hand, taking no account of themselves and almost losing sight of themselves, were more concerned with providing for others. In short, those who are in straitened circumstances are willing beyond their ability if they spend anything on others from their slender means.