How can one counter-argue this argument against the existence of God?

The argument cited in the Question looks like an elaborated version of the age- old riddle: " Can God create a stone so big that he himself cannot lift ? " . In fact Genesis only says that God saw the things created by him as 'Good' , and not as 'Perfect.' If the universe , with all the living beings in it, has survived so far, it is because it has incessantly been rejuvenating itself through the process of living and dying. One cannot just wish away death if one wants the universe to be rejuvenated. Agreed that it is sinful for human beings to kill one another. But, animals have formed a cycle of consuming one another for food, as a part of their programming. So, if God allowed his creation to die in order to pave way for newer generations , and let His Only Son die for ensuring eternal life of humankind, it should only be seen as an outcome of His Infinite Wisdom, something incomprehensible by human standards !

Personally I like the reasoning of Leibniz. In short, his argumentation goes like this: God is omnipotent, so He can do whatever is possible. He is also good, so He created the world as good as possible. So, whatever we may not like about the world, whatever we would call “less than perfect” about the world, still is the best possible world.

The argument is not undisputed. It has been ridiculed by other philosophers and for a Christian it may be a bit problematic to accept the idea that even God can not do the impossible. But it is an interesting philosophical idea, I think, that resembles things you can find in Holy Scripture. For example, when Job calls God to answer for all the wrongs God has done to Job. God doesn’t explain himself to Job, He doesn’t stand trial. On the contrary, He questions Jobs right to call God to answer for Himself.

In a way, that is what Leibniz says in a more modern philosophical way: we can think something is evil or wrong in the world, and that is Gods fault, but who are we to question God? If this is the world created by God, we must assume and accept that this is the world as it should be, the best possible world.

(see also: https://www.britannica.com/topic/best-of-all-possible-worlds )

The argument never addresses the possibility God intentionally created a world that suits him. It leaves this option completely open. It just so happens this is the result Christians claim. Furthermore, the omission is meant to be obvious. Clearly we live in a flawed and broken world, and how could a perfect and powerful God allow that? This actually attacks the Christian worldview as so absurd as to not even be worth mentioning or considering.

A more modern way to view this restates it like this, to state the issue explicitly:

If God is good, he is not God. And if he is God, he is not good.

The existence of cancer in children is also often brought up as a common case-in-point. A child has not sinned. How could a perfect God allow such a thing to happen to a sinless person? He must be either powerless to stop it or a participant in making it happen. Ultimately, this is the problem of how we deal with suffering.

My answer has to come in two parts: one to address the specific and one to address the abstract.

I'll start with the specific. While I can go several directions making abstract arguments for the general problem, that's not helpful in the moment. For the person dealing with suffering right now, I don't want do anything other than be present with them and offer comfort. It's important to make that plain first. When suffering happens, all the other lofty logic we might produce is counter-productive and calloused.

Having stated this, the context we have right now is more of an abstract question, inviting us to respond on that axis at length. Still, I prefer brevity and I'll reframe the problem one final time in order to keep this short. We can reduce it down to this:

Why didn't God create a perfect world?

It is fundamentally the same issue as first posed, and we can answer like this: "He did. Twice." The world as it was first created was perfect. It was only after mankind brought sin into the world that we had any death or suffering. A child has cancer not because of their sin, but because they were born into a corrupted and sinful world. One could argue this itself is a flaw, but the Christian can respond even this was also part of God's plan from the beginning, that the world will be redeemed and anyone who wishes can be with God in Heaven, where there will be no sin, no suffering, and the joy there is so far greater than any previous suffering as to render it meaningless.

I would ask him first, "How do you know that God is not perfect? What he is doing is making "an argument from silence." One cannot make a positive assertion of truth based on what is NOT said.

In short, his argument is "fallacious." There are a number of verses that claim God is perfect. Psalms 18:30, Deuteronomy 32:4 and one more, 2 Samuel 22:31.

Even Jesus Christ is identified as perfect and sinless according to 1 Peter 2:22, and at John 1:14, "And the Word became flesh, and dwelt among us, and we beheld His glory, glory as of the only begotten from the Father, FULL OF GRACE AND TRUTH." Ask him to reconcile his opinion with the Scriptures.

Even as a heathen atheist and cultural Jew I can see the problem with this. Any argument based on "God is perfect" presumes that we're able to judge what perfection is. The argument you quoted is based on an arbitrary definition of perfection deliberately chosen so that it would not be consistent with some actions that God purportedly took.

I believe logicians would call this a strawman argument.

Mark had a good answer, to which I will add a separate angle.

The word "perfect" can mean essentially two things in English:

1. Flawless/pure; and
2. Complete/finished.

It appears to me that the question conflates these, claiming or implying that because God's creation was not completely finished (#2), God is not without flaw (#1). But these are two different things, and the question itself is what is flawed.

Consider: A "perfect" child Jesus was not yet "complete" according to Luke 2:52, for he continued to grow, learn, and develop.

And Jesus increased in wisdom and stature, and in favour with God and man. (Luke 2:52, KJV; cf. vs. 40)

Development (increasing) does not imply imperfection.