Is thinking of God as a Loving Father specific to Christianity? I was looking to respond to someone who sent me a Christopher Hitchens video saying that nothing would be different if Jesus never existed and I was thinking of all the ways this is dead wrong.  One of those ways was that Jesus taught to address God in Heaven in our prayers as "Our Father".  This may have been foreshadowed in the OT, as far as I know it was never made explicit until Jesus did so.  

So for the purposes' of apologetics, can I say concretely that the idea of God as a loving Father is specific to Christianity?
It is not specific to Christianity, there are many other religions that use similar associations of deities with male or female parents. This is a well-known form of anthropomorphism in the human sciences: the attribution of human traits, emotions, or intentions to non-human entities. It is considered to be an innate tendency of human psychology.


In the Norse pantheon, one of the names of Odin is Alföðr, literally "all-father", father of all.
Similarly, in the Greek pantheon, Zeus was respected as an allfather who was chief of the gods. Even the gods who are not his natural children address him as Father. He was also called the father of men.
In Native American Animism, there is the All-Father, the Sky-Father, and the Earth-Mother
The Egyptian God Khnum bears the title "father of fathers" in some of the oldest religious preserved texts.
In Babylonian, Sumerian and Akkadian texts we find the titles "father of all" and "father of heaven" for the deity Anu.


The principle of associating the top deity of any pantheon with "father" was very common in older patriarchal societies. 

It is also not the case that "Jehovah as father" was never made explicit in the OT:


  Jer 31:9  They shall come with weeping, and with supplications will I
  lead them: I will cause them to walk by the rivers of waters in a
  straight way, wherein they shall not stumble: for I am a father to
  Israel, and Ephraim is my firstborn.
  
  Isa 64:8  But now, O LORD, thou art our father; we are the clay, and
  thou our potter; and we all are the work of thy hand.
  
  Isa 63:16  Doubtless thou art our father, though Abraham be ignorant
  of us, and Israel acknowledge us not: thou, O LORD, art our father,
  our redeemer; thy name is from everlasting.
  
  Deu 32:6  Do ye thus requite the LORD, O foolish people and unwise? is
  not he thy father that hath bought thee? hath he not made thee, and
  established thee?
  
  1Ch 29:10  Wherefore David blessed the LORD before all the
  congregation: and David said, Blessed be thou, LORD God of Israel our
  father, for ever and ever.
  
  Jer 3:19  But I said, How shall I put thee among the children, and
  give thee a pleasant land, a goodly heritage of the hosts of nations?
  and I said, Thou shalt call me, My father; and shalt not turn away
  from me.
  
  Mal 1:6  A son honoureth his father, and a servant his master: if then
  I be a father, where is mine honour? and if I be a master, where is my
  fear? saith the LORD of hosts unto you, O priests, that despise my
  name. And ye say, Wherein have we despised thy name?
  
  Mal 2:10  Have we not all one father? hath not one God created us? why
  do we deal treacherously every man against his brother, by profaning
  the covenant of our fathers?
  
  Pro 3:12  For whom the LORD loveth he correcteth; even as a father the
  son in whom he delighteth.
  
  Psa 103:13  Like as a father pitieth his children, so the LORD pitieth
  them that fear him.