Does something need to have proof demonstrated for it to be true? If I say "I love my wife!" but you don't buy it... does that mean it is not true?
You have to go back to Medieval Philosophy.
Under the influence of ancient Greek philosophy, European medieval theologicians developed the "project" of applying the tools of rational inquiry to the "science of God".
To be able to rationally prove the existence of God was aimed to have a very powerful argument to contrast atheist and other people "lacking of the true faith" (Jews, Muslims,...).
Proof consists of two factors:
- Burden of Proof the obligation to prove one's assertion
- Evidence the available body of facts or information indicating whether a belief or proposition is true or valid
You have made the assertions: "God exists" and "I love my wife". The obligation lies on you to prove them to my satisfaction.
As it stands these are simply statements, they might be true and the might be false. Now, the statements are objectively true irrespective of whether you or I believe them to be true: God exists or God doesn't exist; you love your wife or you don't. There is an interesting question which we leave to the side of if the statement "I love my wife" can be true or false if you don't actually have a wife!
A basic tenant of philosophical argument (which flows into our legal systems) is that the person who makes the assertion is obliged to prove it. In the absence of such proof the assertion is unproven.
So this now brings us to the question of what constitutes evidence. Well, you have the burden of proof so you have to provide the evidence. You have to convince me so I have to decide if I find your evidence creditable and persuasive.
Further, we have to agree on how much I have to be persuaded by your evidence. Proof is not a binary proven or unproven but a continuum running from "Absolutely" through "I'm pretty damn sure", "Seems pretty likely", "More likely than not", "Could go either way", "Less likely than not", "Seems unlikely", "You're kidding, right?" to "No way in the world". This is known as the threshold of proof.
For example, in the Australian legal system, the threshold of proof for a criminal trial is "Beyond reasonable doubt" while for a civil trial it is "On the balance of probabilities". The difference here is that the former requires that that no other logical explanation can be derived from the facts except that the defendant committed the crime while the latter simply means that one side has more evidence in its favour than the other.
For the question of the existence of God you might present as evidence:
- The existence of the Universe
- The existence of the planet Earth which is so eminently suited to life
- The existence of life
- The existence of human beings
- The existence of God's miracles
- The existence of income tax
I might not accept that your evidence is creditable because I have alternative explanations for these phenomena. Alternatively I might accept some and reject others or I might accept all of them. I would then need to decide based on the evidence that I accept if you have reached the required burden of proof: that is, to what extent have I been persuaded by your evidence.
Why does one need 'proof' that God exists
Not everyone does. Some people accept God's existence without evidence. From above they will accept anything that meets the "No way in the world" burden and above.
What constitutes 'proof'?
Does something need to have proof demonstrated for it to be true?
No, facts are true or not independent of evidence or belief. More complex concepts may be partially true and partially false, or true in some times and places and not in others, or both true and false simultaneously.
If I say "I love my wife!" but you don't buy it... does that mean it is not true?
No, that means you haven't proven it to me. Alternatively, we may have different ideas about what "love" means.