The answer from God’s word:
“The 6,000 years of work of God’s management are divided into three stages: the Age of Law, the Age of Grace, and the Age of Kingdom. These three stages of work are all for the sake of mankind’s salvation, which is to say, they are for the salvation of a mankind that has been severely corrupted by Satan. At the same time, however, they are also so that God may do battle with Satan. Thus, just as the work of salvation is divided into three stages, so the battle with Satan is also divided into three stages, and these two aspects of God’s work are conducted simultaneously.