What does Paul mean when he says that in the past God let all nations go their own way?

In the past, God permitted nations to continue with their idolatry; he did not reveal himself in a special way to every nation. The only nation which received special revelation was Abraham and his descendants (Israel);¹ they were in the sphere of God’s mercy.² Every other nation was left with the testimony of general revelation only (see Psalm 19:1–6; Romans 1:19–23). Now, with the proclamation of Christ (special revelation), the period of ignorance has come to an end.³