IHS or IHS is used a lot in Catholic liturature.  Some people allege it comes from the phrase "In Hoc Signo vinces" or "in this sign you will conquer", but it was in the Bible long before Constantine.  Others claim it is the first letter of 3 pagan dieties.

The 3 letters actually come from how Yeshua's name appears in ancient Greek and Latin manuscripts of the Bible.  If you look at an old Majiscule manuscript, it looks a lot different than a modern interlinear of Greek.  The letters are written in all capitals, no spacing between words, and the sacred names are written as abbreviations.  "IHSOUS" is abbreviated "IHS" or "ΙΗΣ" in Greek.  Some manuscripts used "ΙΣ" instead.  "IHSOUN" is abbreviated "IHN" in Greek.  "KURIOS" is abbreviated "KU".

If you want to see this for yourself, consult almost any Majiscule manuscript and examine Matthew 4:17, 9:2, 9:4, 9:22, 9:23, 11:1 or 11:2 for just a few examples.  One place you can see this is http://cudl.lib.cam.ac.uk/view/MS-NN-00002-00041/ which has Codex Bezae online.  You can see it at http://upload.wikimedia.org/wikipedia/commons/4/4b/Papyrus_5_-_Oxyrhynchus_1781_-_British_Library%2C_inv._2484_-_John_1%2C_20.jpg , 2nd line from top of bottom most piece.

IHN was another common abbreviation used in Scripture.  You can find it in Matt 14:29, 17:8, 26:4 to name just a few places.  IHU is another common abbreviation (Matt 26:6 and 26:17).  IHS, IHN, IHU and IHE are the 4 Greek case ending abbreviations used and Latin adds the possibility of IHM, and I think one or two others.  The nominative or subject form of a name is what is typically used in abstract references, so a reference to IHS would be expected in an abstract reference.

One of the reasons I am aware of this is because I have indeed examined a lot of these ancient manuscripts.  People who don't know Greek or Latin well enough to examine ancient manuscripts for themselves sometimes fall prey to the idea that perhaps there is something sinister to this.  But there is nothing bad about it.  It is simply a linguistic tradition found in Scripture.

This was done in almost all early Majiscule manuscripts from the earliest times.  In later centuries when scribes switched to miniscule writing and putting spacing between words, the Latin IHS and Greek ΙΗΣ dropped out of use as the Name became spelled out in completion in later years.

Now the Catholic Church did indeed bring a number of pagan practices into Catholicism , mixing Sun worship with Son worship , such as choosing December 25 to celebrate the birth of Yeshua.  That date was in use by ancient sun-god worshippers to celebrate the rebirth of the pagan sun-god.  And because of that, I don't honor that date because I don't want to get caught in the paganism that Rome tried to bring into Christianity.  But the use of IHS is rooted in ancient Latin and Greek manuscripts, not in anything pagan.

Shalom, Joe