Here are the fallacies I can spot : 

> X: Aliens do not exist **because God** did not mention them in the Bible.

There are four fallacies in the previous quote : 

The statement in bold was an **Appeal to the God (or heaven) fallacy**, What deities did or said never contributes to a good argument, after all, ancient fanatics could turn sacrificing children into an act that is ethically right if they were to commit this fallacy.

The second fallacy is an **Appeal to Authority**, even if God has the authority to say such and such, that does not make Him/Her/It right. We have never met a deity and we do not know how Gods behave, or whether they lie or say the truth, or whether they are good, evil or neutral (if they do exist).

The third fallacy is an **Appeal to the Scripture**, if x is written in the scripture, it does not mean it is from God or that it is right. Scriptures never contribute to good arguments.

The fourth fallacy is formal and is a **Denying the Antecedent** , I assume person 1 would agree with the conditional `if God mentioned A in the Bible then A exists`, but they would be a fool if they agree with the conditional `if A exists then God mentioned A in the Bible`, since there are many things that exist, and that we know exist, which are not in the Bible (like planes and computers).

So, I would take person 1 seriously and assume they are not a fool, in this case they agree only with the conditional `if God mentioned A in the Bible then A exists`. And I would give the propositional form of their argument :

>  - **Premise 1 (implied):** If God mentioned Aliens in the Bible then Aliens exist
>  - **Premise 2:** God did not mention aliens in the bible
>  - **Conclusion:** Therefore, aliens do not exist

If P then Q , Not-P therefore Not-Q : This was an invalid argument.

Of course, either Person 1 is a fool to think that if x exists then it's in the Bible, or they are not a fool and they have just committed a **denying the antecedent fallacy**


> Y: You can't say that because **maybe God mentioned them in some other
> book that is not in the Bible.** For example, **there is the Book of Enoch
> that talks about God punishing angels who sinned on earth.** Did you
> know **that God did that?**

The first statement in bold is an **Argument from Ignorance**, or *Ad Ignorantiam* if we want to sound a bit smarter.

Also, there is an **appeal to authority, to the scripture and to God**, since the questions asks : **did you know that God did that?** and not **Did you know that it is written in the book of Enoch?**

So, the question `did you know that God did that?` implies this conditional : `if it is written in the book of Enoch then God did that`.

> So, how do you know that God did not mention aliens in one of **those
> books that are not in the Bible** or said it in a book that has **not yet
> been discovered?**

What is in bold is a clear **Appeal to the ignorance**.