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

You seem to be missing the singular point which is the crux of Salvation.

Salvation is not believing in God. Salvation comes from believing that Jesus in dying on the Cross paid our sin debt in full.

To the best of my knowledge all faiths that are known as Christian believe that it was Jesus death and resurrection that provides for our being able to get to Heaven in which no evil can enter.

Genesis 8:21 KJV And the LORD smelled a sweet savour; and the LORD said in his heart, I will not again curse the ground any more for man's sake; for the imagination of man's heart is evil from his youth; neither will I again smite any more every thing living, as I have done.

We cannot live a life which does not include sin, since Satan has cloaked sin in what appears to be desirable to man. He has used that same ruse from Eve in the Garden to us today including when he tempted Jesus himself.

Believing in God will not get you into the Kingdom of God:

Galatians 1:3 through 5 KJV Grace be to you and peace from God the Father, and from our Lord Jesus Christ, Who gave himself for our sins, that he might deliver us from this present evil world, according to the will of God and our Father: To whom be glory for ever and ever. Amen.

The Jewish Judaists do not accept the Divinity of Jesus, and as a matter of fact that was the very reason the Sanhedrin wanted Jesus dead.

John 5:17 and 18 KJV But Jesus answered them, My Father worketh hitherto, and I work. Therefore the Jews sought the more to kill him, because he not only had broken the sabbath, but said also that God was his Father, making himself equal with God.

And that was what they found him guilty of at their illegal trial.

Luke 22:67 through 71 KJV Art thou the Christ? tell us. And he said unto them, If I tell you, ye will not believe: And if I also ask you, ye will not answer me, nor let me go. Hereafter shall the Son of man sit on the right hand of the power of God. Then said they all, Art thou then the Son of God? And he said unto them, Ye say that I am. And they said, What need we any further witness? for we ourselves have heard of his own mouth.

From the way your question is worded it is difficult to know if you believe in a singular Being as God or a tripartite God. God as a Trinity is three equal Spirits one of which at one point assumed a human form, and each of these beings has a separate, function.

God the father is the seat of power, that is from that Spiritual being the other two equal Spiritual beings draw their power to perform his will:

John 5:19 and 20 KJV Then answered Jesus and said unto them, Verily, verily, I say unto you, The Son can do nothing of himself, but what he seeth the Father do: for what things soever he doeth, these also doeth the Son likewise. For the Father loveth the Son, and sheweth him all things that himself doeth: and he will shew him greater works than these, that ye may marvel.

God the son Provided a path for salvation for sinful man, and sits at the Right hand of the Power acting as our advocate.

Matthew 26:64 KJV Jesus saith unto him, Thou hast said: nevertheless I say unto you, Hereafter shall ye see the Son of man sitting on the right hand of power, and coming in the clouds of heaven.

1st John 2:1 and 2 KJV My little children, these things write I unto you, that ye sin not. And if any man sin, we have an advocate with the Father, Jesus Christ the righteous: And he is the propitiation for our sins: and not for ours only, but also for the sins of the whole world.

God the Holy Ghost (Spirit) Is our teacher and mentor.

Luke 12:11 and 12 KJV And when they bring you unto the synagogues, and unto magistrates, and powers, take ye no thought how or what thing ye shall answer, or what ye shall say: For the Holy Ghost shall teach you in the same hour what ye ought to say.

MarK 13:11 KJV But when they shall lead you, and deliver you up, take no thought beforehand what ye shall speak, neither do ye premeditate: but whatsoever shall be given you in that hour, that speak ye: for it is not ye that speak, but the Holy Ghost.

Jesus told us to Baptize not in the name of God but:

Matthew 28:19 KJV Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost:

If God is one then why is belief in God insufficient?
Faith in God is indeed sufficient, it is however misunderstood because faith falls into three categories.

Jesus said to her, “I am the resurrection and the life. He who believes in Me, though he may die, he shall live. 26 And whoever lives and believes in Me shall never die. Do you believe this?” (John 11:25-26 NKJV)

Therefore there are Three Categories

• Category 1: Those that Believe in Jesus
• Category 2: Those that Live and Believe in Jesus
• Category 3: Those that Do Not Believe in Jesus

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?
This question is a proof that the definition of "believing" is not "believeing that there is a Jewish/Christian God". For logically if "belief" was "believeing that there is a Jewish/Christian God" then indeed all Jews would have already been saved, and the "Need for Jesus's Ministry" would not have been nessecary.

Now this will be a summary because it is not the Focus of the Question

Understanding Faith
Now that we "Know" the "Knowledge of Good and Evil". When a person recieves "The Knowledge of Evil(Error/Mistake/Wrong)" for example "Finding out they are fired" The "Power of Sin" is in "Unbelief" that leads to Anger, Bargaining, Depression, and Axiety. So a change in ones Mood(Spirit) takes place. Therefore Faith is when "The Knowledge of Evil" is realized that "Trust" in God to "Repair the Situation" or "Make a Benifit of the Situation" will preserve the Good Mood (Holy Spirit). Like wise when a person recieves "The Knowledge of Good(Truth/Correct/Right)" for example "Recieving Praise a Job Well Done" belief in the "Self" leads to "Pride of Accomplishment" instead of "Praise to God realizing that the self is only His Temple".

Category 1: Those that Believe in Jesus
Those that believe in Jesus recieve the "Holy Classifications of the Attutides/Moods(Spirit)" so the Spirit when the person Dies will come to Life when the Jesus Calls. [Romans 8:11]

Category 2: Those that Live and Believe in Jesus
Those that live and believe in Jesus recieve the Holy Spirit, and correctly remain in the "Words of Truth/Water of Life" correctly applying the First Commandment to "Have No Other Gods Before the Face of the Word". {Exodus 20:3, John 1:1}

Category 3: Those that Do Not Believe in Jesus
Those that Do Not Believe in Jesus never recieve the Holy Spirit so are subject to "Adultery, fornication, uncleanness, lasciviousness, Idolatry, witchcraft,hatred, variance, emulations, wrath, strife, seditions, heresies, Envyings, murders, drunkenness, revellings, and such like." {Galatians 5:19-21}

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?

The Logical Presentation to the Parts and Singularity of God

God is Spirit, and those who worship Him must worship in spirit and truth.” (John 4:24 NKJV)

The Meaning to the Word "Spirit"
The Greek Word is {Πνεῦμα} pronounced Pneuma means: spirit (Spirit), wind, or breath

The Wind

The earth was without form, and void; and darkness was on the face of the deep. And the Spirit of God was hovering over the face of the waters. (Genesis 1:2 NKJV)

The Breath

And all flesh died that moved on the earth: birds and cattle and beasts and every creeping thing that creeps on the earth, and every man. 22 All in whose nostrils was the breath of the spirit of life, all that was on the dry land, died. (Genesis 7:21-22 NKJV)

And when He had said this, He breathed on them, and said to them, “Receive the Holy Spirit. (John 20:22 NKJV)

And the Lord God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living being. (Genesis 2:7 NKJV)

And they went into the ark to Noah, two by two, of all flesh in which is the breath of life. (Genesis 7:15 NKJV)

And when Jesus had cried out with a loud voice, He said, “Father, ‘into Your hands I commit My spirit.’” Having said this, He breathed His last. (Luke 23:46 NKJV)

The Spirit of God has made me, And the breath of the Almighty gives me life. (Job 33:4 NKJV)

So the Father "Is the Breath"{John 4:24} which is Also the Spirit{John 4:24}, and the "Son of the Breath" is the "Word" who is God.

In the beginning was the Word, and the Word was with God, and the Word was God. (John 1:1 NKJV)

How to Prove that the Word is the Son of the Breath
Problem Solving Exercise: Try to Speak a Word without Wind/Breath.

Observable Evidence
All things are Made from Atoms: Atoms Vibrate
Visibility is of Light: Light Vibrates
Audible Sound: Sound Vibrates

Therefore of all of the Universe(Creation) is based off differentiating frequencies. The Word of God.

We therefore are inside of the Word of God, and the Breath of Life is inside of us.

Do you not believe that I am in the Father, and the Father in Me? The words that I speak to you I do not speak on My own authority; but the Father who dwells in Me does the works. 11 Believe Me that I am in the Father and the Father in Me, or else believe Me for the sake of the works themselves. (John 14:10-11 NKJV)

Note of personal interest: The Internet that Holds so much Truth is passed to our computers by frequencies. So the Internet is another form of "God Flexing" so I am thankful. Just saying. Anyways.

When Sound raises its Frequency it becomes Light.

Then God said, “Let there be light”; and there was light. (Genesis 1:3 NKJV)

So within the Infinite the Solar Winds(God/Spirit/Universe) the vibrations raised to a frequency that allowed light to become visible.

Conclusion: God is All things Because It Is:

• All Contained With Him
• All Matter was formed from the vibrations (The Word)

Conclusion: God is Inside you (Making your Body a Temple) and what you Speak is "From God"

• I Breath Air that was once part of the Solar Winds - The Air is In Me, and the Air is also God.
• It is the Air that Allows for the Words to Speak - Without the air no vibrations are made.

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

Trying to answer the question how I best understand it

Some starting points are assumed:

1. God exists. (More accurately, God IS [Existence]).
2. He is One.
3. He is True.
4. He is Good.

cf. God | New Advent, The Existence of God | New Advent, and The Nature and Attributes of God | New Advent.

That religion is universal, is plain.

But if there is God, and he is One, and he is True, and he is Good, logically, if he reveals himself to man, there must be at any one time, just one true religion.

Revelation provides the answer: Catholicism is Judaism revealed or Judaism completed or the fullness of Judaism, like caterpillar turned butterfly. This by the way is taken from the remarks of Jews who have come to the fullness of their religion in the Catholic Church (I have purposefully avoided the word 'conversion' as they do not see themselves converting). cf. "Fulfilled Jews".

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?

This is best answered by what Jesus said to the Jews who claimed Moses as their Teacher, yet rejected Jesus.

Jn 5:45-46 (RSVCE)
45 Do not think that I shall accuse you to the Father; it is Moses who accuses you, on whom you set your hope. 46 If you believed Moses, you would believe me, for he wrote of me.

For Catholics, to be saved, one needs to worship God by [the theological virtues of] Faith, Hope and Charity. [cf. Penny catechism, 8.].

And faith is believing whatever [/ALL - answering the question] God has revealed without doubting because he can neither deceive nor be deceived (plus he is True, Good, etc. as above). [cf. Penny catechism, 9. & 10.].