Understanding who he is takes us to 1 John 2:18 KJV where antichrist is introduced to us.
Little children, it is the last time: and as ye have heard that antichrist shall come, even now are there many antichrists; whereby we know that it is the last time.
Recognizing that we are living in these days, we learn from the Greek Manuscripts that antichrist and “Instead of Christ” are connected; “anti’s” use in the word antichrist is “Instead of”:
anti, an-tee'; a primary particle; opposite, i.e. instead or because of (rarely in addition to) :- for, in the room of. Often used in composition to denote contrast, requital, substitution, correspondence, etc.
The manuscripts tell that “anti” and “instead of” are interchangeable and that antichrist and “Instead of Christ” are one and the same. John’s warning in this verse is that “Instead of Christ”, substituting as Christ, is coming.
Knowing that antichrist and “Instead of Christ” are one and the same, we return to scripture for his identity.
Satan, prior to falling from grace, was the archangel Lucifer. Knowing that Christ returns to the Mount of Congregation, Isaiah 14:12-14 KJV shows Satan’s intention. It illustrates him taking the place of Christ. It shows “Instead of Christ” posturing as Christ, claiming to be like the most high:
 How art thou fallen from heaven, O Lucifer, son of the morning! how art thou cut down to the ground, which didst weaken the nations!  For thou hast said in thine heart, I will ascend into heaven, I will exalt my throne above the stars of God: I will sit also upon the mount of the congregation, in the sides of the north:  I will ascend above the heights of the clouds; I will be like the most High.
These Old Testament verses reveal that “Instead of Christ” is Satan and moving forward to the New Testament, 2 Thes. 2:4 KJV serves as a second witness to this truth:
Who opposeth and exalteth himself above all that is called God, or that is worshipped; so that he as God sitteth in the temple of God, shewing himself that he is God.
Understanding that Satan is “Instead of Christ” is important and knowing that he comes first is equally important; 2 Tim. 3:1 KJV recognizes this truth as it introduces these perilous times in which we now live:
This know also, that in the last days perilous times shall come.
These are days that God reaches out to us; He seeks support from His Elect.