If God is one then why is belief in God insufficient? The title is a bit misleading since it's limited in characters so I'll try my best to explain:

Christianity teaches (at least many forms do) that one must attain salvation through acceptance of Christ. Furthermore, there is only 1 God - Christ and God are one in the same.  

If God is one, then why do some teach that Jews for instance, who presumably believe in the same God as Christians, cannot be saved by merely believing in this God?

Doesn't the precondition that one must specifically accept one "form" (I apologize if I am misusing terminology) of God suggest that God is not in fact singular?

To put it the other way around, if God is one, then purely by a logical tautology, why isn't accepting any or all of his forms sufficient for salvation?
Simply put, If you imagine God as a house, then Jesus is the door - 


  Jesus said to him, "I am the way, and the truth, and the life. No one comes to the Father except through me." - John 14:6 ESV (cf. John 10:1-9)


If you want to go in, you go through him.  The door is part of the house and inextricably linked to (one with) it:


  [ Jesus: ] "I and the Father are one." - John 10:30 ESV (cf. John 14:9, Hebrews 1:1-3)


Once through the door, you have access to the whole house:


  For through him [Christ] we both have access in one Spirit to the Father. - Ephesians 2:18 ESV (cf. Romans 5:2)


But if you seek to go another way, at best you will only ever get glimpses through a window, you will not actually enter in (to true fellowship with the living God by adoption into His family) and more likely you will either be staring at a blank wall or falling in a ditch!


  No one who denies the Son has the Father. Whoever confesses the Son has the Father also. - 1 John 2:23 ESV (cf. John 3:35-36, Romans 8)


Note the 'must' in Acts 4:10-12 - 


  10 let it be known to all of you and to all the people of Israel that by the name of Jesus Christ of Nazareth, whom you crucified, whom God raised from the dead—by him this man is standing before you well. 11 This Jesus is the stone that was rejected by you, the builders, which has become the cornerstone. 12 And there is salvation in no one else, for there is no other name under heaven given among men by which we must be saved. (ESV, emphasis added)


Prior to the New Covenant, God made provision for people to approach him by Faith in His grace and mercy through the ways He showed them to approach him at that particular time (to Abraham it was by believing his word and travelling out in obedience, under the Mosaic Covenant, it was by seeking to be obedient to God's law and exercising faith in the efficacy of the tabernacle/temple sacrifices to atone for sin [which were a shadow of what was to come] when (inevitably!) falling short of God's standard). These means were never meant to be stand alone systems that could provide their benefit independantly of a continued faith in God and obedience to His ways, but they were always contingent on the more perfect means of Salvation that God had prepared to be the substance of their fore-shadowing from all eternity past - which remained veiled until it was finally revealed through the work of the Father's only begotten son - Jesus Christ who represents the clear and infallible (not in any way speculative) way of salvation - if you put your trust in him you must be saved.


  For since the law has but a shadow of the good things to come instead of the true form of these realities, it can never, by the same sacrifices that are continually offered every year, make perfect those who draw near. 2 Otherwise, would they not have ceased to be offered, since the worshipers, having once been cleansed, would no longer have any consciousness of sins? 3 But in these sacrifices there is a reminder of sins every year. 4 For it is impossible for the blood of bulls and goats to take away sins.
  5 Consequently, when Christ came into the world, he said,
  “Sacrifices and offerings you have not desired,
      but a body have you prepared for me;
  6 in burnt offerings and sin offerings
      you have taken no pleasure.
  7 Then I said, ‘Behold, I have come to do your will, O God,
      as it is written of me in the scroll of the book.’”
  8 When he said above, “You have neither desired nor taken pleasure in sacrifices and offerings and burnt offerings and sin offerings” (these are offered according to the law), 9 then he added, “Behold, I have come to do your will.” He does away with the first in order to establish the second. 10 And by that will we have been sanctified through the offering of the body of Jesus Christ once for all.
  11 And every priest stands daily at his service, offering repeatedly the same sacrifices, which can never take away sins. 12 But when Christ had offered for all time a single sacrifice for sins, he sat down at the right hand of God, 13 waiting from that time until his enemies should be made a footstool for his feet. 14 For by a single offering he has perfected for all time those who are being sanctified. - Hebrews 10:1-14 ESV (emphasis added)


In summary: You asked - 

If God is one then why is belief in God insufficient?


The short answer is that belief in God is entirely sufficient if by that you mean trusting him entirely and accepting the perfect way He has shown us to approach Him and fellowship with Him (i.e. Through Christ); If you don't trust in the way He has provided though (especially if you actually reject it!), the (orthodox/Nicaean) Christian perspective is that you are not actually believing (i.e. trusting and obeying) God

So well we might say:


  how shall we escape if we neglect such a great salvation? - Hebrews 2:3a ESV