Personally I agree with the spirit behind the statement “all truth is God’s truth”. It’s just a way of saying that we should respect the truth wherever we find it and whoever is speaking. I think where I would be careful, for example with Buddhism, is that sometimes when people saying one thing - such as life including pain - they mean much more. The entire worldview is connected to that one statement - so that it cannot really be taken just at face value.
I think religions tend to have this type of connected truth more often than science. Scientists generally work in a fairly closed domain or specialized field - so when they come to a conclusion it is not necessarily connected to every other branch of science or (necessarily) to a worldview. When dealing with philosophical or religious statements, I think I would be a bit more careful because of the way people connected one idea to another.
It’s like an iceberg - you see the one statement they made - life involves suffering - but beneath the surface icebergs are massive and so are people’s intended meanings sometimes. That one statement - life involves suffering - may include concepts of maya and the eight fold path and karma.
And with your example of Mary from the Quran - that statement assumes that the Quran is a reliable source of information. Which is a separate claim entirely from the statement about the virgin birth.
So I think that would be my personal caveat - beware the strings tied to a statement that appears to be true when you view it from your own worldview.
I read the article linked below - it had some interesting quotes that I listed below the link.
However, I think both Calvin and Bavink, in these quotes, make claims that I find unnecessary. Calvin says that the " the Spirit of God is the only fountain of truth" and Bavink says that God is “the truth in all truth”. Bavink’s statement sounds almost eastern - God is not in truth - God is the Creator and Sustainer of all things - truth points to Him, but saying He is ‘in truth’ is a bit odd. Calvin’s point, to me, is slightly off. The Gospel and truth about God are spiritually discerned, but a math problem - 2+2=4 - does not require God’s Spirit to figure out - there are different types of truth.
John Calvin - Therefore, in reading profane authors, the admirable light of truth displayed in them should remind us, that the human mind, however much fallen and perverted from its original integrity, is still adorned and invested with admirable gifts from its Creator. If we reflect that the Spirit of God is the only fountain of truth , we will be careful, as we would avoid offering insult to him, not to reject or condemn truth wherever it appears (II.2.15)
The Dutch Reformed systematic theologian Herman Bavinck wrote the following: “He [God] is the truth in its absolute fullness. He, therefore, is the primary, the original truth, the source of all truth , the truth in all truth. He is the ground of the truth – of the true being – of all things, of their knowability and conceivability, the ideal and archetype of all truth, of all ethical being, of all the rules and laws, in light of which the nature and manifestation of all things should be judged and on which they should be modeled. God is the source and origin of the knowledge of truth in all areas of life…”