That's absolutely correct. All MP's must swear either an oath of allegiance or an affirmation of allegiance. This is done at the beginning of the Parliamentary session following the general election.
The text for an oath is:The current standard oath of allegiance is set out from the Promissory Oaths Act 1868 in the following form:
I, (Insert full name), do swear that I will be faithful and bear true allegiance to Her Majesty Queen Elizabeth II, her heirs and successors, according to law. So help me God.
This is done holding the Holy Book of their own choice. I have witnessed this event many times and it is always interesting to see what is chosen. The person can choose a Holy Bible King James Version, an Old Testament (if they are a Jew), the Koran (which is kept inside a little bag) or some other religious texts. I have witnessed many non-Christians swear using the Bible, so it doesn't necessarily have to reflect the person's personal belief- sometimes it may be for other reasons such as tradition.
Alternatively, if the person is a secularist or atheist etc they can choose to simply affirm. The text is as follows: "I, do solemnly, sincerely and truly declare and affirm that I will be faithful and bear true allegiance to Her Majesty Queen Elizabeth II, her heirs and successors, according to law."
I have witnessed several of the Welsh MP's swear in the welsh language, usually followed by the English. Likewise, some of the SNP's swore in Gaelic (no pun intended!) and I believe one in Cornish.
The Sinn Fein MP's who are voted into Parliament will not swear or affirm as they refuse to bear allegiance to the Queen. This is why Sinn Fein MP's are not allowed into the House of Commons Chamber.