I have wondered at the origin of the phrase. I remember something about the axes of executioners being reassuringly inscribed with it. But I didn't know for certain. Spoiler alert: I still don't. I did learn that most executioners' swords were not pointed but squared off. There's no use putting a point on a sword meant for chopping.
Today there was an exchange in the catbox which caused my audible laughter:
<IWhoSawTheFace> Put your Hay-und own the Jet Fuel In-JECT-torrs!
The Custodian punches 'IWHO' into his GPS with intent
<The Custodian> CAST IN THE NAME OF GOD YE NOT GUILTY
I still have little idea what was going on, but Custo's comically violent words haunted me into employing the most powerful and rigorous form of research immediately available to me: a Google search.
I was surprised. I found a few museum/civic sites with descriptions of swords in their collections/histories, some threads on forums, and a couple 1990s-style historical information pages. It seems that it was quite common for executioners' weapons to be enscribed with a maxim. However I found no saying which so directly absolved the wielder of his neck-chopping ways, excepting on the fictional sword belonging to Conan.
The major themes I encountered are listed below in bold. Sources with many inscriptions are not linked in full in the listing, but are cited at the very bottom under "Sources". If you have any correction or contribution, /msg me!
Many thanks to Chad Reasco and Clockmaker for their help translating the Latin.
Fictional
Cast in the name of God, ye not guilty.
(displayed during the boot sequence of a few giant robots)
Big O
Suffer no guilt, ye who wields this in the name of Crom.
Conan the Barbarian
Mercy
Wen ich das Richtschwerdt wohle Gott gnad der armen Seele. (1709)
(On whom I use this sword of justice, God give grace to his poor soul.)
goantiques.com
God's mercy and faith are renewed every morning. Raising this sword, I help the sinner find eternal rest.
Estonia Tourism site
When I raise my sword on high may God give the poor sinner eternal life.
Swords Used By Executioners[1]
No Mercy
Ich schone niemand. (1729)
(I spare no one.)
sbg sword forum
Warning
When I raise the sword I wish the poor sinner eternal life. / Keep from evil doing if thou wouldst escape this judge.
Swords Used By Executioners[1]
If thou act in the service of virtue this sword shall not strike thee. (1324)
Swords Used By Executioners[1]
Authority / Necessity / Justice
Whenever I raise this sword I wish eternal life to all these pitiable sinners. My masters impede evil and I carry out the sentence. (1602)
Official Site of the City of Trebic, Moravia
Ich Muß straffen daß verbrechen / Als wie Recht und Richter sprechen
I must punish crime as the law and judge tell me.
weapons-universe.com (and a better German transcription here)
Si Deus pro nobis, quis contra nos?
(If God is for us, who is against us?)
fitzmuseum.cam.ac.uk
This sword is whetted and I am deputed by God and the authorities to punish wicked people.
Swords Used By Executioners[1]
Viva la Justice. (1721)
Swords Used By Executioners[1]
Honor
Omnia si perdas, famam servare memento. / Verbum domini, manet in aeternum. (circa 1600)
(If you are to lose everything, at least preserve your honor. / The word of the Lord endures forever.)
artancient.com
Soli deo gloria.
(Glory to God alone.)
Swords Used By Executioners[1]
Fide sed cui vide soli deo gloria.
Have faith but understand in whom; only glory to god.
Swords Used By Executioners[1]
Do not draw me without cause. / Do not sheathe me without honor. (WWII Japan)
a blogger's trip to a museum in Charleston, SC, USA ** Not an executioner's sword.
Sorrow
Posledm nessastud prdce Ityla dne 21 cervna 1621
"The last sad work was on 21st June, 1621." (accompanied by a list of martyrs' names)
Archaeology Data Service (PDF)
See Also
thread about a sword inscription project
Sources
[1] SWORDS USED BY EXECUTIONERS. New York Times, August 15, 1886. (Full article is a PDF.)
tanktop says re Cast in the Name of God, Ye Not Guilty: Interesting research. Thanks for sharing it. People have also inscribed bombs, and made invocations when using weapons. "Praise the Lord, and pass the ammunition." Cheers.