Meaning of _gläubig_ in this context

Question

There is a famous and often quoted sentence by Albert Einstein, which reads:

Für uns gläubige Physiker hat die Scheidung zwischen Vergangenheit, Gegenwart und Zukunft nur die Bedeutung einer wenn auch hartnäckigen Illusion.

It was written in a condolence letter to the family of his deceased friend Michele Besso, and short before Einstein's own death.

I'm in doubt about the exact meaning of gläubig here. In general, if I understand correctly, this adjective may refer either to someone believing in a specific religion, or to someone who is trustful in something, not necessarily of a religious character. In different translations into English or Italian, I find uns gläubige Physiker translated in different ways, mostly “us believing physicists” and “us, who believe in physics”.

Is there any element – as far as possible internal to the text and to German language and as little as possible resorting to other Einstein's texts and what we generally know about him – that can actually lead us to understand that this occurrence of gläubig is to be considered as a secular concept rather than a religious one?

EDIT: Source for the quote: Pierre Speziali (Hrsg.): Albert Einstein – Michele Besso. Correspondance 1903–1955, Hermann, Paris 1972 (the standard edition of Einstein-Besso letters, with the German text and the French translation); the letter quoted here is dated 21 March 1955. The quote itself appears in several other books, for instance here and here. On Christie's website there is a reproduction of the manuscript letter.

Answer 1

For a more detailed answer please take a look at "Die Zeit – eine Illusion?" by Hans Joachim Störig.

Störig writes

Was meint er mit dem Ausdruck „gläubige Physiker“? Eins ist sicher: Bestimmt meint er nicht Gläubige im Sinne der jüdischen oder der christlichen Religion. Dem Christentum hat Einstein niemals nahe gestanden. Vom jüdischen Glauben seiner Vorfahren hat er sich ausdrücklich losgesagt.

What did the expression "believing physicists" mean? One thing is certain: He certainly does not mean believers in the sense of the Jewish or Christian religion. Einstein was never close to Christianity. He expressly renounced the Jewish belief of his ancestors.

So as Störig says it is no expression of "gläubig" in the classic (christian/jewish) religious way.

But

In seinem Innern war Einstein jedoch ein tief religiöser Mensch.

Inside, however, Einstein was a deeply religious person.

Einstein's answer to the question: "Do you believe in God?" was

"Ich glaube an Spinozas Gott, der sich in der gesetzlichen Harmonie des Seienden offenbart, nicht an einen Gott, der sich mit den Schicksalen und Handlungen der Menschen abgibt."

"I do believe in Spinoza's God, who reveals himself in the legal harmony of beings, [but] not in a God who deals with the fates and actions of men".

Störig concludes

Der Ausdruck „gläubige Physiker“ zielt offenbar nicht auf die Religion, sondern meint Menschen, die die Erkenntnisse der modernen Physik – zu denen Einstein das Kernstück beigesteuert hat – zur Grundlage ihres Weltbildes und ihrer Lebensanschauung gemacht haben.

The expression "believing physicists" apparently does not refer to religion, but means people who have made the knowledge of modern physics - to which Einstein contributed the core - the basis of their worldview and their view of life.