‘And you know that because of an infirmity of the flesh I preached the Gospel to you the first time (or ‘previously'), and you did not despise, nor did you reject with loathing (literally ‘spit out') that which tested you out in my flesh. But you received me as an angel of God, even as Christ Jesus.'
It is clear that Paul had been suffering some kind of severe infirmity when he first went to the Galatian region with the Gospel, an infirmity that had made it advisable that he go there rather than elsewhere. Malaria has been suggested, so that he had to seek higher ground. In that region such fevers were often seen as demonstrating divine anger, and may thus have been regarded with loathing. But they had not despised him. Another possibility is epilepsy or even some kind of putrefying sore, or pus in the eyes, for which he sought medical aid in the region (perhaps Luke was there), and which thus prevented him from travelling any further. But the important point is that they took it without flinching and even welcomed him warmly. They were not put off by his illness.
‘You did not despise, nor did you reject with loathing.' They might have suggested that God had smitten him, and thus have mocked him, or they might have loathed what they saw and turned away from him. He acknowledges quite freely that they might well have been tempted to do so and that it was a genuine test of their goodwill. But they had not.
‘You received me as an angel of God, even as Christ Jesus.' See Acts 13:44; Acts 13:48; Acts 14:1; Acts 14:11-12. Rather they had treated him like a divine messenger (at Lystra literally), yes, they could not have greeted him more warmly had he been Jesus Christ Himself. He may even mean that they mistook him for the Messiah Jesus, but the warmth of his words suggests that he was speaking about more than just a mistake and therefore is suggesting that what he means is that they greeted him with the same warmth they would have shown to Jesus Christ.