Moral systems rooted in theism are preferable to non-theistic moral systems.

First off I would like to preface that this is my first debate on this platform so I am probably unfamiliar with the customs and how debates are conducted here. Nevertheless I hope that I can provide a fun experience.

On contention 1: while having the morals written down may be useful there is still the issue of interpretation. Disagreements between how holy books should be interpreted would further increase confusion and as is evident by history these disagreements tend to turn violent. 

On contention 2: I do not see how belief in common sense and basic human decency can't be grounded in logic and reasoning. To say belief in anything is a belief in Theism because you believe it's important I think is quite the overstep. Logic and reasoning can provide sufficient grounding for most things. Additionally this detracts from the idea that things are good simply because of their nature and that things are only important due to a god. One can view human life as important without any reference to a god, for example it is in my best interest to preserve the human race as I am a member of it, thus the view that an act that preserves human life is moral seemingly circumvents the need for a god by appealing to logic and reasoning.

On contention 3: I see no reason why the first part "multiple people having a shared moral system is beneficial in building community" could not also apply to a moral system based on logic and reasoning. While Theism may strengthen connections between those of similar beliefs it may also alienate those of different beliefs and lead to conflict. Supposing both sides to be Theists but of different religions both would declare the other side to be completely immoral as they ground their morals in the incorrect text.

(Don't actually know what you call the objections to your point so I'll just call them objections)

Objection 1- Morals presented in Theistic holy books are barbaric and go against logic and reasoning: I'll use Christianity as an example but this can also be applied to other holy books. The Bible is riddled with genocide(1 Samuel 15:3), homophobia (leviticus 18:22), misogyny (1 Timothy 2:11-15) and slavery(leviticus 25:44-46). I'm sure you and I both agree we would not want to live in a society where this kind of stuff is morally acceptable.

Objection 2-holy books are seen as infallible and unchangeable: as holy books are seen as the word of God the moral claims are seen as absolute fact. This may lead to stagnation in societal growth due to morals not evolving. For example a society with its morality based solely on the Bible would never accept homosexual relations as it is seen as a "detestable act". However morals based on logic and reasoning leave room for growth, previous thinkers can be criticised and their ideas thrown out for more favourable ones. This cannot happen if holy books are infallible.

Objection 3-Theistic moral system can lead to an increase in discrimination and hate against those who don't follow that religion: This one seems pretty self explanatory, if 2 groups both believe that they have the one true source of morality then conflict is bound to arise. If morals were based on logic and reasoning it is much easier to solve these kinds of conflicts and they are less likely to escalate to violence.

Objection 4- science disproving the metaphysical claims of religion may undermine the moral claims: I'll once again use Christianity as an example. We know for a fact that the Earth was not made in 6 days so this undermines the trustworthiness of the Bible as a scientific source. An undermining of a holy book as a scientific source may also lead to the morals of the book being undermined as it is seen as less trustworthy leading to confusion as to what is true and what is false.

Objection 5- Theisms methods of enforcing moral behaviour are flawed: the idea that one should be good in order to receive eternal joy and one shouldn't be bad in order to avoid eternal punishment appeals only to human selfishness and detracts from doing the right thing simply because it is right.

If I've formulated my argument in the incorrect way or missed anything please let me know. Any pointers would be greatly appreciated.