Many people are surprised to learn that in many places in the Hebrew Scriptures, the word sa·tan´ appears without the definite article (the). Used in this way, it refers to individuals as resisters of other men. (1 Sam. 29:4; 2 Sam. 19:21, 22; 1 Kings 5:4; 11:14, 23, 25) But when it is used with the definite article (ha), it refers to Satan the Devil, the chief Adversary of God. (Job 1:6; 2:1-7; Zech. 3:1, 2) In the Greek Scriptures 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.
How He Became Satan The Devil
Deut. 32:4 mentions that God's ‘activity is perfect’ with no injustice or unrighteousness. Therefore, the one becoming Satan was, when created, a perfect, righteous creature of God. Jesus Christ showed that Satan was once in the truth, but abandoned it. 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; 1 John 3:8)
So just as a formerly honest man makes himself a thief by stealing, one of the perfect spirit sons of God long ago acted upon an improper desire and made himself Satan the Devil. The Bible explains the process: “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)
When God created Adam and Eve, the angel who was about to rebel against God (Satan) saw that there was a possibility that he could gain honor and importance. He evidently wanted worship from humans...what rightly belongs only to the Creator. Instead of rejecting this improper desire, this spirit son of God nurtured it until it gave birth to a lie and then to rebellion. By means of a serpent, he then spoke to, lied and deceived Eve. (Genesis 3:1-5)
How He Got the Name 'Satan the Devil'
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. As was previously mentioned, the word “devil” means “slanderer” and “Satan” means “Resister,” or opposer. So in order to determine between individuals as resisters of other men or Satan the Devil, the chief Adversary of God, depends on the presence or absence of the definite article (the).
By his challenge of God and his charging God’s servants with lack of integrity, Satan lived up to his title “Devil,” meaning “Slanderer,” which title he deserved for having slandered God in the garden of Eden. The name he had before this is not mentioned in the Scriptures.
(The name Lucifer occurs once (Isa. 14:12) in the Scriptures and only in some versions of the Bible. Traditionally, Lucifer is a name that in English generally refers to the Devil before being cast from heaven. But this description is given to a man and not to a spirit creature as is further seen by the statement: “Down to Sheol you will be brought.” Sheol is the common grave of mankind—not a place occupied by Satan the Devil. Additionally, those seeing Lucifer brought into this condition ask: “Is this the man that was agitating the earth?” Clearly, “Lucifer” refers to a human, not to a spirit creature. - Isaiah 14:4, 15, 16. Also see: "Is Satan Lucifer?" (Jimspace)
The Scriptures Portray Satan The Devil as a Real Person
The Devil is clearly a spirit person, for he appeared in heaven in the presence of God. (Job chaps 1, 2; Rev. 12:9) Because he is a spirit creature he therefore is invisible to humans for the same reason that God is invisible to human eyes. “God is a Spirit,” says the Bible. (John 4:24)
Some contend that Satan is simply the "personification of the evil." However, the Biblical evidence showing that Satan is a real person include the instances where Satan actually CONVERSED with others. (Genesis 3:1-5) God Himself talks to Satan at Job 2:1-7 and Jesus does too at Luke 4:3-12. Satan cannot be the "personification of the evil" since God and Jesus do not have any evil at all in them.
Another instance showing how Satan is a real person is the account at Rev. 12:7-9, where Michael and his angels actually waged war with Satan and had him "hurled down to the earth". This would make no sense if Satan was merely the "personification of the evil".
For much more, see the articles listed below:
SATAN (Insight-2 pp. 866-870; Watchtower Online Library)
The Bible’s Viewpoint - Who Is Satan? Is He Real? (JW.ORG)
THE BIBLE’S VIEWPOINT - Is Satan a real person? (JW.ORG)
Who is Satan the Devil? (Search For Bible Truths)
Is Satan a real person? (Search For Bible Truths)
How did one of God's spirit sons become Satan the Devil, and what was his motive? (Search For Bible Truths)
Why is Satan and his angels invisible to our eyes? (Jehovah's Witnesses Questions and Answers)
Is Satan really the ruler of the world? (Search For Bible Truths)
What are scriptural indications that show that Satan is ruling the world? (Jehovah's Witnesses Questions and Answers)
Was Satan around at the time of Adam and Eve and was he responsible for the snake? (Search For Bible Truths)
Where in the Bible does it show that Satan is a spirit being? (Jehovah's Witnesses Questions and Answers)
Did Satan have more power than Michael according to the account at Jude 9? (Search For Bible Truths)
Is Satan the Devil able to read our minds? (Search For Bible Truths)
Do all of our desires and temptations come from Satan and his demons? (Jehovah's Witnesses Questions and Answers)