First off, thanks for a civil and well written post

I'll do my best to respond in kind.
Well, I would first have to ask, how can we determine something to be objectively good or bad? What is objective and what is subjective? Of course something that is objectively true will be true regardless of opinion. Even if it was unanimously agreed to be true does not make that thing objectively true. Take for example the shape of the earth. It is objectively true that it is round and not flat. So even if the entire world population agreed that the earth was flat, everyone would be objectively wrong. Now, lets hypothetically say that the earth had no shape but rather a blob that morphed! What shape would it be then? Some may say it is round and others may say it is flat. However, everyone's statement would be subjective to the individual's point of view. Thus, nobody would be objectively correct.
"Good" and "Bad" are judgments that are made about something, and all judgments by definition are subjective. The idea of something being objectively good, or objectively bad makes no sense. Even if a god existed, his labelling of something as good or bad is still the subjective viewpoint of that god.
He may be better informed than we are, however it's still ultimately a subjective view that god would hold.
With all this being said, how can we define something to be "objectively good/bad in a way that would be true regardless of unanimous opinion? I would argue that it would depend entirely on "created purpose". Now, that purpose does not have to be from a god or gods (I use "god or gods" because this argument can apply to any god and not just the God of Abraham). The only thing required for something to have a "created purpose" is for a being with some level of intelligence to create something for a specific purpose. I think we all can agree that nothing has ever been created for a purpose without a being of intelligence bestowing a purpose onto that creation.
There are two problems with this statement:
1) While I would agree that nothing has ever been created by a non intelligent being, you have not demonstrated that we were created. You're making an unsupported assumption, it's possible we could have arisen from natural processes.
2) If you can address point 1, then you also have not demonstrated that this creator had any purpose in mind. Things can be and are created with any particular purpose in mind all the time.
For example, I could go into a workshop and cut a random piece of wood into a random shape, just because. I have no particular reason to do so, however if I did, then I would have created something without any particular purpose in mind.
Therefore this claim is also unsupported. You need to demonstrate that this creator had a particular purpose in mind for us.
Lets take a knife and a rock for example. If the purpose of a knife is to cut, it would be objectively true that a good knife is one that is sharp and strong so that it cuts well. It would be objectively bad if a knife was dull and weak because it cannot cut well. We can determine this because the knife has a "created purpose" that was bestowed upon it by it's creator.
Well, not necessarily. A dull knife still has uses, even if it was not the original intent. A dull knife can still cut through butter, it can still be used to pry open things, depending on the shape of the blade you can tighten screws with it, etc.
It would be objectively true that a dull knife doesn't cut as well as a sharp knife, however again, good or bad is still subjective. If I'm trying to cut through butter, a dull knife is still good.
However, a rock was created by nature by natural means. So how do we define what a good/bad rock is? I may pick up a rock and say it is good because it is shiny and colorful and is good for collecting. Someone else may say it is good because it is smooth and good for skipping across the water. Others may say it is bad because it is not hard and flat so you cannot build upon it. Now, given that I am a being with some level of intelligence, I can bestow a purpose on something that has no created purpose. I can take a rock and put it on top of a stack of papers and call it a paperweight. Depending on how well it can keep paper from blowing away will determine how good/bad my rock paperweight is.
Two issues with this paragraph as well:
1) Isn't it true in your worldview that god created everything? Would that rock also not have a purpose then? This would not be a result of random natural processes, this would be the intelligent design of your creator being.
2) If you are capable of bestowing purpose on a rock that has no created (or inherent) purpose, then what would prevent human beings from bestowing meaning and purpose on ourselves in the absence of a created purpose?
So, how does this apply to humanity and how does this determine what is an objectively good/bad person is? Well, in order for mankind to have been created with a "created purpose" it would require that a being with some level of intelligence (god, gods, or superior alien race) to have created mankind with an intended purpose. Thus, anything that fulfilled that intended purpose is "objectively good" and anything that went against that created purpose is "objectively bad".
In fulfilling an intended purpose, perhaps. However that doesn't necessarily say anything about a persons character or morality.
For example, say we were seeded here by an intelligent alien race to be a species of genocidal monsters that they'd use to help conquer the galaxy, and instead we build a peaceful and harmonious civilization, then we'd be objectively bad as per the intent of our design.
I'd call us a good and moral species however.
So, I think you're conflating the "functional" meaning of good (i.e. the thing works well, therefore it is good) with the "moral" meaning of good (i.e. They are a good person)
If a god or gods do not exist and we are nothing more than a creation of nature by natural means without a created purpose, we will be no different than a rock and thus a good/bad does not exist. However, given that we are beings with some level of intelligence, we can bestow a purpose onto ourselves. The only problem is that purpose is subjective. Are we a rock made for skipping over water, adding to a collection, or laying a foundation for a home?
Well, I dispute that we're no different than a rock, as we are conscious beings and a rock is not... However from the standpoint that everything is a product of nature, sure.
And why is it a problem that purpose is subjective?
Even if god exists and bestowed a purpose onto us, the reasons for bestowing that purpose is still subjective. I'll explain:
Pretend god designed you with the purpose of being the best mathematician in the world, and you'd help advance the human race forward technologically. While it may be true that your objective purpose is to be the best mathematician in the world, it's also true that god subjectively decided that he would design you with that purpose in mind. It was his will that you would be the best mathematician, and a conscious will or desire is subjective in nature.
That can be further demonstrated by god having the ability to change his mind. Say you annoyed god and he no longer wanted you to be the greatest mathematician in the world, presumably he would have the power to either weaken your abilities, or create an even better mathematician that would scoop all of your intended discoveries before you had a chance to figure them out. If that's the case, then you are still subject to god's subjective whim.
So ultimately even if god does impose a purpose upon us, that purpose is determined by god's subjective will.
So even if god imposes a purpose on us, or we bestow purpose upon ourselves, either way it's still rooted in a subjective decision to do so. One is god's subjective decision, the other is ours.
If I decide my purpose in life is to become the greatest mathematician in the world, then it is objectively true that I have bestowed that purpose on my own life. It's exactly the same situation as if god made that decision.
So the argument is this, If objective morality in humanity exist, an intelligent being must exist who created humanity for a specific purpose. If there is not an intelligent being who created humanity for a purpose, objective morality cannot exist. But rather, all morality is subjectively based on the individual purpose that we bestow unto ourselves.
I don't think fully objective morality could exist regardless of if a god exists or not. As I explained above good and bad are subjective judgments (as all judgments are subjective by definition), and that's true even if your god was making those judgments as well.
Secular morality does have an objective basis that god based moral systems don't necessarily have though, and that's the objective consequences to our actions. Consequences are what they are regardless of our opinion on them, therefore they are objective in nature.
For example, feeding someone a lethal dose of poison will kill them, that is an objective fact. (We'll assume this person is an otherwise healthy person that has no desire to die, just to avoid the grey area arguments like euthanasia. Likewise, killing this person will not save any other lives, or anything like that)
You are causing a great deal of harm to that person for no good reason. You'd have to redefine what the term morality means in order to not call that act immoral. We can reach that conclusion by examining the objective consequences of the act. That is a true objective basis for moral judgments.
Likewise, if you help a little old lady across the street, you are (obviously) helping that person. You are doing good things for people, with no discernable negative consequence. Therefore we have good reason to call that a moral act.
Furthermore, I'd argue this is exactly how you determine right from wrong as well. Would you help a little old lady across the street because god wants you to do it, or would you do it because it's the moral thing to do? And would the existence or non existence of a god have any bearing on the moral implications of your act?
I should clarify however that the judgment is still subjective, however we do actually have an objective basis to work from. Consequences are objective.
Here is the problem with this argument. I does not prove the existence of God whatsoever. We have no way of determining which god or gods created mankind with a purpose. Since unanimous consensus plays no factor on determining something to be objectively true (flat/round earth example) there is no way to know for sure.
I completely agree with this point.