Classical Theism

Hi everyone!

A 17 year old agnostic theist here. Knowing some of WLC's arguments and the ilk but extremely little about philosophy and classical theism in overall.
Also excuse if you see some distasteful grammar and spelling, i'm not a native english speaker myself.

So here is the question: It's often said that theists can define God in any way that suits them. Lacking empirical evidence and observation of God further strengthens/ is the foundation of this claim.
So what is your response to this? And how would one solve it?

Thank you very much!

I don't think that theists can define God in "any way" that suits them. I myself am committed to a more thomistic understanding of God, but much of it would overlap with common understandings of God.

An important thing to keep in mind is that you do  not need to fully and perfectly understand or comprehend a concept in order to use it meaningfully.

Most people share a common, intuitive understanding of what "God" means. They don't think it is literally an "invisible old man on a cloud" or anything silly like that. Rather, the common, intuitive idea of what God is supposed to be would be something like that:
'The Creator of the universe and all contingent reality... an Eternal, Necessary, Uncaused, Self-Sufficient, Immaterial, Spaceless, Intelligent, Wise, Good, Personal Creator... All-powerful, all-knowing, benevolent..." etc. This understanding is good enough, for starters. Philosophical arguments can establish the existence of such a being.

For example:

Leibnizian and Thomistic Cosmological arguments (they show there is a First Cause of reality, and it is Necessary, Purely Actual, Self-Sufficient and Eternal. It is also immaterial, since material beings are contingent, dependent, etc. The First Cause is also very powerful, intelligent because of various different arguments, etc. So, God);
The Kalam Cosmological Argument (shows there is an Eternal creator of the physical universe. It is immaterial since it is outside of space and time. It is immensely powerful. Also plausibly personal because of different arguments. So, God);
The Fine-Tuning Argument (shows there is an intelligent mind behind physical reality who favored order and life; it is immensely powerful since it governs physical laws; it is immaterial since it basically transcends the universe and controls it, etc. So, God);
Arguments from Consciousness and Mind (shows there is an immaterial Creator of our minds and consciousness, etc. Plausibly God, again);
Moral arguments (shows there is a Perfect, Transcendent being who is the ground of all goodness and ordained reality and moral laws, etc. Basically God, again).

And so on.

So, despite whatever disagreements there might be, there is a common, intuitive understanding of what "God" is, and it is the sort of being whose existence can be reached through different arguments of natural theology.

I recommend you to read some introductory books on this subject. If I may recommend, check these out:

"How reason can lead to God" by Joshua Rasmussen. This book is really, really great. It is very didactic. The writer is an expert professional philosopher and the whole book is an extended argument for the existence of God. Please give it a try and read it carefully, you will enjoy it.

"Who designed the designer?" by Michael Augros. This book is very simple, and also very didactic. It defends a classical thomistic argument for the existence of God. Also give it a shot, you will really enjoy it if you read it with good attention.

These two books are very simple and easy to read, and pack a lot of content.