The Christian/Jewish/Muslim God exists in the Buddhist universe.  Why didn't he try to spread dhamma? (8 fold path).

He had prophets and their teachings are not dhamma.  There are even Devas in heaven who practice dhamma.  God has to know the truth of it all.

I'm not asking as a Christian who needs reassurance.  I'm asking as a Buddhist who knows the Christian God exists in the Buddhist Universe and I see a big conundrum.

I know there are some obscure Christianity practices that seem to point to attainments but you think there would be more than that.

Maybe read the philokalia to get a new (or really, very old) perspective on the bible.

I have what I think a simple solution to your dilemma. It has two steps:

1) The universe is not Buddhist. People can be Buddhist, but the universe, not being a person, does not have religious faiths or philosophical opinions.

2) The Christian god, Thor, Voldemort, Satan, Frodo, Zeus, R'hollor, Spiderman, Allah, Cthulhu and Ganesha are human inventions. The universe does not care about them, because it does not read books.

Contradiction solved! If anyone has a better solution, I am all ears.

You have wrong views. The advaitins would say that the personal god rises and disappears from and in the Brahman, free of any attributes, like ice solidifying from water. 

The new testament is part of the Dhamma. But the bible lacks when it comes to practice instructions. 

People never hear of the Dhamma or practice it after they hear it due to lack of wisdom, infatuation, laziness, karmic tendencies.  Sankara says, having a male body, being born in a religious family and having an inclination to study and live by the scriptures is the fruit of the good deeds done in a million past lives.

So there, most people don't hear of the Dhamma because of their karma.

In Buddhism there are many gods, not just one as in monotheistic religions like Christianity. According to the Buddha all gods are not free because they delight in creating, which in turn binds them to existence as a god. And all existence is related to Mara. The Buddha claimed he broke free from this sort of existence. He claimed his understanding (awakening) was superior to all the existing religious beliefs of his day and those of any god. He even claimed to have once been the ruler of all the gods, but it was only in a human existence that he broke free. At least that is how the story goes.

So another way to answer your question, god didn't create Nibbana, doesn't know it, and cannot teach the path toward attaining Nibbana; only the Buddha did so.

So there isn't a conundrum within Buddhism itself, as you thought there was. But I can see how a monotheistic religion which places God at the top would see in Buddhism such a conundrum, but the Buddha sorted this out by placing his understanding on top.

I never heard this one, interesting. Would you have a link to the original source? Is it in the Pali Canon?

Being the ruler of gods is in Itivuttaka: Iti 22 which is presented by Khujjuttar

This was said by the Blessed One, said by the Arahant, so I have heard: "Monks, don't be afraid of acts of merit. This is another way of saying what is blissful, desirable, pleasing, endearing, charming — i.e., acts of merit. I am cognizant that, having long performed meritorious deeds, I long experienced desirable, pleasing, endearing, charming results. Having developed a mind of good will for seven years, then for seven aeons of contraction & expansion I didn't return to this world. Whenever the aeon was contracting, I went to the realm of Streaming Radiance. Whenever the aeon was expanding, I reappeared in an empty Brahma-abode. There I was the Great Brahman, the Unconquered Conqueror, All-seeing, & Wielder of Power. Then for thirty-six times I was Sakka, ruler of the gods. For many hundreds of times I was a king, a wheel-turning emperor, a righteous king of Dhamma, conqueror of the four corners of the earth, maintaining stable control over the countryside, endowed with the seven treasures[1] — to say nothing of the times I was a local king. The thought occurred to me: 'Of what action of mine is this the fruit, of what action the result, that I now have such great power & might?' Then the thought occurred to me: 'This is the fruit of my three [types of] action, the result of three types of action, that I now have such great power & might: i.e., giving, self-control, & restraint.'"

Being born as a human is in SN 56.48 Chiggala Sutta: The Hole

"It's likewise a sheer coincidence that one obtains the human state. It's likewise a sheer coincidence that a Tathagata, worthy & rightly self-awakened, arises in the world. It's likewise a sheer coincidence that a doctrine & discipline expounded by a Tathagata appears in the world. Now, this human state has been obtained. A Tathagata, worthy & rightly self-awakened, has arisen in the world. A doctrine & discipline expounded by a Tathagata appears in the world.

There's also this old story of a monk who wanted to understand "where the 4 elements cease without remainder". He asked the devas, and they didn't know, so he visited a series of higher level devas, kings and gods (including Sakka, mentioned in the post above this one) until he reached the Great Brahma. Even he couldn't answer the monk, and sheepishly told the monk to ask the Buddha instead.

Also interesting, not only did the Great Brahma not know the answer, he didn't even realize the question was phrased incorrectly. He also was slow to admit he didn't know the answer, since the devas were so certain of his omniscience and he didn't want to admit failure. The story seems to illustrate that gods are capable of the same sort of ignorance that binds us, and don't necessarily understand the dhamma.

http://www.suttas.com/dn-11-kevaddha-sutta-what-brahma-didnt-know.html

It's possible that different cultures teach different things, and religion is part of culture.

What's the difference between "God" and "(Inter)Dependent Origination"?