Why does God allow Satan to be in heaven accusing believers?
Pastor Armstrong addresses this question in our Revelation Bible study in Chapter 12. We recommend you listen to that lesson to wholly understand the answer.
In short, the enemy has been allowed access to the throne room for a time, as mercy to the saints. In the future when the Lord finally bars Satan from Heaven once and for all, the enemy will become enraged and direct his anger at humanity:
The Lord casts the enemy out of Heaven three and a half years prior to Christ’s Second Coming, at the midpoint of Tribulation. The enemy’s casting out leads to the worst period of turmoil on Earth in its history.