because of an illness: Paul might have had a long-term health issue, but we do not know what it was (see 2 Corinthians 12:7). Paul's condition was serious and seemed to cause problems or even upset the new church in Galatia. People often saw physical problems as a sign of God's judgment (see John 9:1–2; Acts 28:4). The Galatians looked beyond Paul's physical state and accepted him as God's messenger (literally angel; Greek angelos).
as if I were Christ Jesus Himself: See Luke 10:16.