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:

Rev. 12:7  And there was war in heaven, Michael and his angels waging war with the dragon. The dragon and his angels waged war,
Rev. 12:8 and they were not strong enough, and there was no longer a place found for them in heaven.
Rev. 12:9 And the great dragon was thrown down, the serpent of old who is called the devil and Satan, who deceives the whole world; he was thrown down to the earth, and his angels were thrown down with him.
Rev. 12:10 Then I heard a loud voice in heaven, saying,  “Now the salvation, and the power, and the kingdom of our God and the authority of His Christ have come, for the accuser of our brethren has been thrown down, he who accuses them before our God day and night.
Rev. 12:11 “And they overcame him because of the blood of the Lamb and because of the word of their testimony, and they did not love their life even when faced with death.
Rev. 12:12 “For this reason, rejoice, O heavens and you who dwell in them. Woe to the earth and the sea, because the devil has come down to you, having great wrath, knowing that he has only a short time.”

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.