A fair question at this point would be: If Satan was thrown out of Heaven, how can he be our accuser before God when he is no longer there? But, we find in the book of Job, the first chapter, verses 6-12, that Satan still has limited access to God and will until the end of this age.
6 One day the angels came to present themselves before the LORD, and Satan also came with them.
7 The LORD said to Satan, “Where have you come from?” Satan answered the LORD, “From roaming throughout the earth, going back and forth on it.”
8 Then the LORD said to Satan, “Have you considered my servant Job? There is no one on earth like him; he is blameless and upright, a man who fears God and shuns evil.”
9 “Does Job fear God for nothing?” Satan replied.
10 “Have you not put a hedge around him and his household and everything he has? You have blessed the work of his hands, so that his flocks and herds are spread throughout the land.
11 But now stretch out your hand and strike everything he has, and he will surely curse you to your face.”
12 The LORD said to Satan, “Very well, then, everything he has is in your power, but on the man himself do not lay a finger.” Then Satan went out from the presence of the LORD.
Job 1: 6-12 (NIV)
Although Satan does not have open access to God, he is permitted into Heaven when he is summoned by God. His visits are brief, to the point, and focused on his persistent false accusations against the believers on earth.