Paul calls to their mind their first encounter with him, when his physical infirmity could have been a social stumbling block for them, but they received him warmly and without hesitation. He is drawing a contrast between their first meeting and their current rejection of his teaching.
Paul says that they received him "as an angel [messenger] of God." In the beginning of the epistle, part of his stern warning was against any other doctrines from what he had originally taught them, which included a warning against the teachers of such doctrines (Galatians 1:8). Since the Galatians "received [him] as an angel," they may have likewise received a false teacher as a messenger from God and bestowed upon him the same warmth and acceptance.
