Paul's earlier writing refers either to 1:9–10 (compare 2:11–22) or to an unknown letter.
Paul understood God's mysterious plan because God revealed it to him (see study note on 1:8–10; 1:9–10; Galatians 1:11–12, 15–17; compare Acts 9:3–6; 1 Corinthians 12:1, 7; Galatians 2:2).