That Satan (meaning resistor) is real and an unseen spirit is seen in several Biblical accounts. In many places in the Hebrew Scriptures, commonly called the Old Testament, the word sa·tan´ appears without the definite article. Used in this way, it applies in its first appearance to the angel that stood in the road to resist Balaam as he set out with the objective of cursing the Israelites. (Num 22:22, 32)
In other instances it refers to individuals as resisters of other men. (1Sam 29:4; 2Sam 19:21, 22; 1Kings 5:4; 11:14, 23, 25) But it is used with the definite article ha to refer to Satan the Devil ("Devil" meaning slanderer), the chief Adversary of God. (Job 1:6; 2:1-7; Zec 3:1, 2) In the Greek Scriptures, commonly called the New Testament, the word sa·ta·nas´ applies to Satan the Devil in nearly all of its occurrences and is usually accompanied by the definite article ho.
The Scriptures indicate that the creature known as Satan did not always have that name. Rather, this descriptive name was given to him because of his taking a course of opposition and resistance to God. The name he had before this is not given. God is the only Creator, and ‘his activity is perfect,’ with no injustice or unrighteousness. (Deut 32:4) Therefore, the one becoming Satan was, when created, a perfect, righteous creature of God. He is a spirit person, for he appeared in heaven in the presence of God. (Job chaps 1, 2; Rev 12:9)
Jesus Christ said of him: “That one was a manslayer when he began, and he did not stand fast in the truth, because truth is not in him.” (John 8:44; 1John 3:8) Jesus here shows that Satan was once in the truth, but forsook it. Beginning with his first overt act in turning Adam and Eve away from God, he was a manslayer, for he thereby brought about the death of Adam and Eve, which, in turn, brought sin and death to their offspring. (Rom 5:12) Throughout the Scriptures the qualities and actions attributed to him could be attributed only to a person, not to an abstract principle of evil. It is clear that the Jews, and Jesus and his disciples, knew that Satan existed as a person.
So, from a righteous, perfect start, this spirit person deviated into sin and degradation. The process bringing this about is described by James when he writes: “Each one is tried by being drawn out and enticed by his own desire. Then the desire, when it has become fertile, gives birth to sin; in turn, sin, when it has been accomplished, brings forth death.” (James 1:14, 15) In the course that Satan took, there seems to be, in some respects, a parallel with that of the king of Tyre as described in Ezekiel 28:11-19.
The Scriptural account, therefore, makes it plain that it was Satan who spoke through the medium of a serpent, seducing Eve into disobedience to God’s command. In turn, Eve induced Adam to take the same rebellious course. (Gen 3:1-7; 2Cor 11:3) As a consequence of Satan’s use of the serpent, the Bible gives Satan the title “Serpent,” which came to signify “deceiver”; he also became “the Tempter” (Matt 4:3) and a liar, “the father of the lie.”(John 8:44; Rev 12:9)
Up until Satan and his demon's ouster when Jesus becoming king of God's kingdom in 1914, they had free access to heaven and even the presence of God.(Job 1 & 2) When his disciples returned from having cast out demons, Jesus said: "I began to behold Satan already fallen like lightning from heaven."(Luke 10:18) However, Revelation 12:9 says following Jesus enthronement in 1914 and a subsequent war between Michael ( Jesus heavenly name, meaning "Who is Like God") and Satan, that "So down the great dragon was hurled, the original serpent, the one called Devil and Satan, who is misleading the entire inhabited earth; he was hurled down to the earth, and his angels were hurled down with him."
Hence, Satan and his demons no longer have access to the heavenly realm, but are restricted to the earth, having "fallen like lightning from heaven". As a result, Revelation 12:12 says: "On this account be glad, you heavens and you who reside in them! Woe for the earth and for the sea, because the Devil has come down to you, having great anger, knowing he has a short period of time.”
We are living deep into the "last days" or "time of the end", and therefore Satan has greater anger than ever before, for he realizes that he a very "short period of time" to continue seducing the "world" (1 John 5:19) and to try to ' swallow ' up the "woman's seed", those who are faithful anointed Christians that are observing "the commandments of God and have the work of bearing witness to Jesus."(Rev 12:17)