Why did God banish Satan to earth?
I think the passages you have in mind are as follows: Luke 10:18;Revelation 12:12
The Satan of popular Christian mythology (i,e. a fallen angel) doesn’t exist. Satan is the transliteration of a Hebrew word which means ‘adversary’ The only living adversary to God is man himself who has the propensity, singularly and collectively, to do evil.
In the New Testament the word personifies man’s opposition to God. The heaven(s) refers to the political heaven(s) of man’s rulership over his fellow man.
The ‘Satan’ who ruled in the times of Jesus and his apostles were the pagan Romans. That Satan was 'cast out' of the political heavens when Constantine legalised Christianity and Theodosius outlawed idolatry. This change was brought about by the preaching of the Gospel. The victory is described in Revelation 12:11.
This of course was not the end of ‘Satan’ in the sense that sin still rules in the world to this day. When Christ returns to establish his kingdom and rule over the mortal population of the earth for 1000 years, Satan will be ‘bound’ in the ‘bottomless pit’ – a state of non-existence – for that time. This because the evil which men do will be tightly controlled – there will be no hiding place for criminal behaviour with rulers who can read minds.
At the end of this time there will be a brief relaxation of rulership which will result in a final showdown as described in ;Revelation 20:7-10
This is figurative language denoting complete destruction. The lake of fire is not to be understood literally anymore than the ‘beast’, the ‘false prophet’ and the ‘devil’ are to be understood as literal entities. It denotes the end of mortal opposition to God. All who live on will be immortal, being made equal to the angels. God will live in them and with them forever. This will be the kingdom of God proper on the earth. Revelation 21:3,4
May I suggest that you read Do you believe in a Devil? (Please click on the title)
If you then have further questions, please ask.
I hope you find this helpful.
God bless,
Glenn