I don't think that there is any difference between the two words - either their denotation or their connotation.

Religion can be an expression of spirituality - but spirituality is not necessarily expressed in "religious" ways.

I understand religion to be " a set of beliefs concerning the cause, nature, and purpose of the universe, especially when considered as the creation of a superhuman agency or agencies, usually involving devotional and ritual observances, and often containing a moral code governing the conduct of human affairs." (Oxford)

Well 'religiosity' usually has a negative connotation...it implies an empty formal piety without inner inspiration

Religiousness is a healthy thinker, not in practice, but in faith. Accordingly, a religious person has a strong belief. Peoples are religiousness, but rulers are religiosity.

Religiousness is individual, but religiosity is in the public sphere, social.

Well in English there is a slight difference in connotation that may not exist in other languages...religiosity implies showy, ostentatious piety...like the Pharisee who wants everyone to see him praying...of course sometime 'religiosity' is used as a synonym for 'religious' by students for instance...they are straining for what they think is the more pretentious word....I usually cross it out when they do

As Jacinto Jesús Rodríguez Pérez

I am right?

This is a very interesting question with some interesting answers. According to Merriamwebster.com religiosity is a noun and it means religious feelings or devotion. The first known use for this word according to Merriam-Webster was in 1799 by Heather Havrilesky. This is what Heather said: "The whole notion of pushing your physical limits...has obtained a religiosity that's as passionate as it is pervasive." Merriam-Webster did not have anything to say about religiousness, so I asked google.com what is religiousness? Google says that religiousness is an adjective and it means that which is exhibiting religion; pious; devote; or godly. The Bible says in James 1:27: "Pure religion and undefiled before God and the Father is this, To visit the fatherless (orphans) and widows in their affliction, and to keep himself unspotted from the world (refuse to let the world corrupt you)". :)It would seem as if the Bible says, that religion is our duty. It is our holy acts.

This article will be useful for you.

Iddagoda, Y.A. and Opatha, H.H.D.N.P., 2016. Religiosity: Towards A Conceptualization and An Operationalization. Sri Lankan Journal of Human Resource Management.

Thank you Anuradha Iddagoda for the article.

Jumal Ahmad Some of the answers to this related question previously asked on RG might be useful to you:

https://www.researchgate.net/post/what_is_the_difference_between_religiosity_and_spirituality

Thanks for contribution, all your answers really helped me to finish my Thesis. This is my thesis in Bahasa. Book Religiusitas, Refleksi dan Subjektivitas Keagamaan

Once again, thank you for Karl Pfeifer Anuradha Iddagoda Monica Sarai Ramirez Bernard Wills Faith Bays and all of them.

Jumal Ahmad Thanks for the book, Alhamdulillah. Nonetheless credit to Manap et al. (2013), their work clarify a clear foundation on model of religiosity for Muslims and essential principles for such measurement. Even, Mohd. Mahudin et al. (2016) reiterates the same foundations (Iman, Islam and Ihsan) since it is fundamental for Muslim religiosity.

Well, for me, the two terms refer to the same thing.

So far, I've never found that literature makes a big fuss about using those terms interchangeably. Other scholars also can add new terms for that variable if they want.

A proof to back my answer is the use of Allport & Ross' (1967) 'Religious Orientation Scale'. They called the variable 'Religious Orientation'. While you could find (too) many papers use the scale to measure 'religiosity' and 'religiousness'.

Nonetheless, congratulations on finishing your thesis, Jumal Ahmad !

Semoga menjadi gelar yang bermanfaat (for those who don't speak Bahasa Indonesia - I wish that Jumal's academic degree would bring him many benefits).

Also, adding to Johari Bin Abdullah 's point, I agree that Mahudin et al and Manap et al contribute greatly to Muslim religiosity. I also wrote a paper on the topic concerning the difficulties in measuring Islamic religiosity. Among others, because the measurement of religiosity would not be able to consider the sincerety (ikhlas) and intentions to do the deeds (niyah) - two very important elements in Islam determining individual's religiosity.

The paper might interest you.