The National Pledge is recited in schools every day, but not all Singaporeans may know how it came about.

A Facebook post which shed some light on it has been widely shared and discussed since Home Affairs and Law Minister K. Shanmugam put it up on National Day.

As of last night, it had garnered more than 1,200 reactions and more than 360 shares.

Mr Shanmugam posted a photograph of a letter that Mr S. Rajaratnam, then Foreign Minister, had written to Mr Ong Pang Boon, then Minister for Education.

In a post accompanying this piece of history, Mr Shanmugam wrote: "An old letter between two of our founding fathers: Mr S. Rajaratnam to Mr Ong Pang Boon, suggesting text for our pledge."

In the letter, dated Feb 18, 1966, Mr Rajaratnam wrote: "Sorry for the delay in replying to your letter of 2nd February, 1966. Herewith my suggestion for the pledge for your Flag Raising Ceremony."

The suggested text read: "We, as citizens of Singapore, pledge ourselves to forget differences of race, language and religion and become one united people; to build a democratic society where justice and equality will prevail and where we will seek happiness and progress by helping one another."

While the core message of a democratic society united despite racial or religious differences remains the same, there are slight variations from the final version of the Pledge.

For example, Mr Rajaratnam's version had it as "forget differences of race, language and religion", while the final version reads "regardless of race, language or religion".

The letter sparked a discussion in the comments section of Mr Shanmugam's Facebook post.

Facebook user Christie Anna Blu preferred the "forget differences of race" phrasing.

She said: "Today, it feels more like tolerate differences."

In response, user Melvin Sim said he thought it would be impossible to forget differences.

"I also hope that we are not merely tolerating each other but we recognise that we come from different groups, each bringing strengths to the whole and we will forge ahead together regardless of race, language or religion," he added.

Another user, Swee Keow Koh, wrote: "Thank you for sharing this letter, sir. It is another reminder of the tireless work our pioneer leaders were doing to make SG what it is today."

The Pledge was first mooted in October 1965 as an idea to inculcate patriotism, just months after Singapore's separation from Malaysia.

The first two versions of the Pledge were drafted by Mr Philip Liau, adviser on textbooks and syllabuses, and Mr George Thomson, director of the Political Study Centre, respectively.

Mr Ong submitted these drafts on Feb 2, 1966, to Mr Rajaratnam, who then responded with the third version, which Mr Shanmugam shared on Facebook.

This version then went through another round of revisions by the Education Ministry as well as then Prime Minister Lee Kuan Yew before being approved in August that year by the Cabinet.

It was recited on Aug 24 the same year by about 500,000 students in Singapore before the national flag.