When did Jesus assume human nature? Did Jesus Christ, the second person of the Trinity, only assume impersonal (anhypostatic) human nature (body and soul), during incarnation, in which he was born of Mary? Or, did he already have human soul beforehand (possibly since eternity or after creation), and only assume human body during incarnation?

I believe the former is the biblical position. However, recently I heard the latter idea discussed in my circle.

I would like some rebuttal/support from the perspective of Protestant theology or early church councils.
Frankly, I'm having trouble understanding your question. You seem to describe two scenarios or possibilities. 


  Scenario 1: Jesus assumed an impersonal, anhypostatic HUMAN NATURE in the incarnation (which nature you define as "body and soul"). 
  
  Scenario 2: Jesus possessed a human soul prior to the incarnation which was united with a human body in the incarnation. 


My first question, then, is, 


What is an impersonal, anhypostatic human nature?. 


In theological terms, the hypostatic union is defined as the indivisible joining of human and divine in one person. Jesus Christ was in essence both human and divine. Perhaps I am overstating the case when I say that in Jesus Christ, where the divine ended and the human began, or where the human ended and the human began, is difficult if not impossible to say. 

By the very nature of the incarnation, I suggest that Jesus could never have possessed a anhypostatic human nature. When Mary conceived Jesus in her womb, the Holy Spirit of God, not a human father, caused her to conceive. Mary's human egg was united in an asexual way with the very seed of God, resulting in the only pregnancy of its kind, ever.

As I see it, the distinctive aspect of human nature is the union of the corporeal with the incorporeal. Jesus, prior to his incarnation did not possess corporeality. He was, as was his Father, pure spirit. As pure spirit, Jesus did not need a soul to be self-aware. When he assumed flesh, however, the "soulish" part of his being came from Mary's contribution to the conception. 

In other words, I believe that Jesus did not assume human nature, comprised of body and soul, until he was conceived by the Holy Spirit. Prior to conception, Jesus was the eternally begotten Son of the Father, the second person of the Trinity, who dwelt in the bosom of the Father (John 1:18).

My second question is, 


Why and in what way do you use the term "soul" twice? 


The first time you use "soul" you define human nature as "body and soul." The second time you use "soul" you describe Jesus as possessing a human soul prior to the incarnation. Can a soul exist without first having a body? I agree with you that human nature can be described as the union of soul and body, incorporeal with corporeal. Soul is--perhaps oversimplifying here--self- awareness, or identity. Spirit, on the other hand, is God-awareness, which as we know can be suppressed in ungodliness and unrighteousness (see Romans 1:18-32). 

So again I ask, "Can a soul exist without first having a body?" I think not. 

What follows, then, is my tentative answer to your question as I have interpreted it. 

A key passage (if not the key passage) supporting the view that Jesus did not possess a human soul prior to the incarnation is Philippians Chapter 2, which reads in part:


  5 have the same attitude toward one another that Christ Jesus had, 
  
  6 who though he existed in the form of God 
   
  did not regard equality with God
  
  as something to be grasped,
  
  7 [line a:] but emptied himself
  
  [line b:] by taking on the form of a slave, 
  
  [line c:] by looking like other men, [Grk "by coming in the likeness of people"]
  
  [line d:] and by sharing in human nature. 
  
  8 He humbled himself,
  
  by becoming obedient to the point of death
  
  – even death on a cross!" (NET)


The NET notes make an interesting comment regarding the four lines of verse 7:


  The expression the likeness of men [in line c] is similar to Paul's wording in Romans 8:3 ("in the likeness of sinful flesh"). The same word likeness is used . . . [both here and there]. It implies there is a form that does not necessarily conform to reality. In Romans 8:3, the meaning is Christ looked like sinful humanity. Here the meaning is similar. Jesus looked like other men (note anqrwpoi), but was in fact different from them in that he did not have a sinful nature. 
  
  This last line of v. 7 (line d) stands in tension with the previous line, line c (“by looking like other men”). Both lines have a word indicating form or likeness. Line c, as noted above, implies that Christ only appeared to be like other people. Line d, however, uses a different term that implies a correspondence between form and reality. Further, line c uses the plural “men” while line d uses the singular “man.” The theological point being made is that Christ looked just like other men, but he was not like other men (in that he was not sinful), though he was fully human.[my emphasis]


From Philippians Chapter 2, then, we see that Christ emptied himself in order to take on something else; namely, the likeness of a human being. Now by "emptying himself" Paul does not mean Jesus gave up his deity. One of the best expressions I've come across which expresses what Christ did in becoming a man is that he veiled his deity with flesh. 

In other words, Jesus appeared like any other man, but in reality he was the God-man. The rights, privileges, and prerogatives of deity he veiled in the normal course of events. However, when given permission, so to speak, from his Father, he would display or demonstrate that divinity in signs and miracles such as turning ordinary water into extraordinary wine, calming the wind and the waves by fiat, or raising people from the dead. 

Changing gears for a moment, In Exodus Chapter 21 and Deuteronomy Chapter 15, we read about a strange custom which was enacted (by God's command) whenever the situation called for it. These passages shed much needed light on the great stoop Jesus took to be born of a woman. From Exodus 21:


  Laws about Slaves
  
  When you buy a Hebrew slave, he is to serve for six years; then in the seventh he is to leave as a free man without paying anything. If he arrives alone, he is to leave alone; if he arrives with a wife, his wife is to leave with him. If his master gives him a wife and she bears him sons or daughters, the wife and her children belong to her master, and the man must leave alone.
  But if the slave declares: ‘I love my master, my wife, and my children; I do not want to leave as a free man,’ his master is to bring him to the judges and then bring him to the door or doorpost. His master must pierce his ear with an awl, and he will serve his master for life (vv.2-6).


From Deuteronomy :


  If your fellow Hebrew, a man or woman, is sold to you and serves you six years, you must set him free in the seventh year. . . . But if your slave says to you, ‘I don’t want to leave you,’ because he loves you and your family, and is well off with you, take an awl and pierce through his ear into the door, and he will become your slave for life (vv.12-17, excerpts).


The application of these passages to Jesus's incarnation become clearer when we bring other verses to bear on them. From Psalm 40 (NASB):


  Sacrifice and meal offering You have not desired;
  
  My ears You have opened [or dug, possibly pierced];
  
  Burnt offering and sin offering You have not required.
  
  Then I said, “Behold, I come;
  
  In the scroll of the book it is written of me.
  
  I delight to do Your will, O my God;
  
  Your Law is within my heart.”


And then from Hebrews 10: 


  So when he came into the world, he said,
  
  'Sacrifice and offering you did not desire, but a body you prepared for me.
  Whole burnt offerings and sin-offerings you took no delight in.
  Then I said, ‘Here I am: I have come – it is written of me in the scroll of the book – to do your will, O God.’ 
  
  When he says above, 'Sacrifices and offerings and whole burnt offerings and sin-offerings you did not desire nor did you take delight in them' (which are offered according to the law), then he says, 'Here I am: I have come to do your will.' He does away with the first to establish the second.


I suggest that the salient factors drawn from each of the above passages provide us with an analogy which lays Christ's incarnation side by side with an ancient Hebrew custom (obviously sanctioned by God through Moses). In analogical fashion, then, 


  
  The master is analogous to God the Father
  The slave is analogous to Jesus who came into the world not to be served, but to serve and ultimately to give his life as a ransom for many (Matthew 20:28; Mark 10:45).  
  The digging/piercing/opening of the slave's ear is analogous to Jesus's
  flesh-and-blood body and his willingness to offer up that body as a once for all sacrifice for sin (Hebrews 7:27 and 9:12). In other words, the writer of Hebrews used the slave's ear in synecdochical fashion: the part of the slave's body--his ear, for the whole of Christ's body in his incarnation and ultimate crucifixion.
  The master, the slave, the slave's wife, and the children born to the slave's wife are analogous the Father, the Son, and the Bride of Christ (i.e., the church, those who by virtue of having received and believed in Jesus have been given the right to be called the children of God, according to John 1:12-13). 
  The very notion of slavery is analogous to Jesus's subservience to his Father and to Jesus's determination to accomplish that will at the cost of his own life (compare the above passage from Hebrews to the words of Jesus in John 8:29 and 17:4). 
  The love which the slave had for his master, for his own wife, and for his own children is analogous both to the love relationship between the Father and Son (John 3:35; 5:20; 14:31), and to the love demonstrated by the Son when he died for a world of lost sinners (Romans 5:8)
  The piercing of the slave's ear with an awl is analogous to Jesus's willingness to serve his Father to the point of shedding blood and then bearing the scars to prove it (see John 20:24-29).
  


The distinctiveness of "human nature," as opposed to the "divine nature," is the union of both the corporeal and incorporeal in one person. "God," on the other hand, "is spirit," Jesus said (John 4:24). Jesus himself is also fully God, however, and prior to his incarnation he is described as "the only begotten of God who is in the bosom of the Father" (John 1:18). 

In other words, prior to "becoming flesh and dwelling among us" (John 1:14), Jesus too was pure spirit, as was (and ever will be) the Father. Discounting "theophanies" or "Christophanies" for the time being (i.e., pre-incarnate appearances of Christ as the "Angel of the LORD") as only temporary manifestations of a divine personage in a human body (e.g., Genesis 16:13; Judges 6:21, 22; 6:11, 14; 13:3, 18, 20, 22; Zechariah 12:8; and 2 Kings 19:35), I feel confident in saying that Jesus did not begin to perform his unique role as the "last Adam" (1 Corinthians 15:45) until he was conceived and then born of the virgin Mary.

Paul refers to Jesus as "the second man . . . from heaven" (1 Corinthians 15:47). By this he does not mean Jesus came to earth from heaven as a man; rather, he means that Jesus came to earth as a "life-giving spirit" (1 Corinthians 15:45) in the form of a man, who after he was born "grew in stature and in wisdom, and in favor with God and man" (Luke 2:52). 

In conclusion, the sacrificial system as it was enacted in the history of the Hebrews served its purpose until the ultimate and last sacrifice came on the scene in the person of the Lamb of God who takes away the sin of the world (John 1:29, 36). What the OT sacrifices could never do, Jesus did with the sacrifice of himself. 

Throughout the OT, however, God made clear that more important than the literal sacrifice of an animal is something internal to the person making the sacrifice; namely, a broken and contrite spirit, heart-felt repentance, and above all, delight in obeying God and doing his will (see 1 Samuel 15:32-33; cf. Psalm 40:6-8). 

What was true then is still true now:


  For You do not delight in sacrifice, 
  
  otherwise I would give it;
  
  You are not pleased with 
  
  burnt offering.
   
  The sacrifices of God are a 
  
  broken spirit;
  
  A broken and a contrite heart, 
  
  O God, You will not despise. 


Jesus gave his all for us. Can we do any less?