Up until the nineteenth century, the word "grimace" also meant a (devilish) work of deception. Goethe frequently and emphatically used it in this sense throughout his Faust: In the first part, when the Professor seals his pact with the Devil in blood without taking himself too seriously, he says: "This is after all nothing but a grimace." In the second part, when Mephistopheles disguises himself as a fool, the statement comes from him: "He is all spruced up, / But his grimace looks so grotesque, that everybody is taken aback."

One way to phrase it.

Faust is saying here that he sees no need for formal confirmation. The gestures/outward appearances are enough.

I thought the quotation below was revealing-especially as Faust's pact is a caricature of what happens at Easter. Fratze DOES mean grimace in my opinion and I do not agree with the disagreements given to Andres. I think his "grimace" is correct but didn't like "this is nothing but.." because that is not in myopinion what Faust is saying. He is stressing that the appearance of intent requires no formal /ceremonial confirmation.

Here is the description of the Fratze in Medieval legend from which Goethe probably drew inspiration for the line in question:

Wer in früheren Zeiten das erstmals 1267 urkundlich erwähnte Ostertor passierte, begegnete dort einer hölzernen Fratze aus dem 17.Jahrhundert, die als Schreckdämon bei jedem Glockenschlag den Vorübergehenden die Zunge herausstreckte.

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Note added at 2 Tage8 Stunden (2010-04-06 11:17:47 GMT)
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I took the trouble to read the surrounding text intensively and concede that Faust regards the entire proceedings including the signing of the document as hocus-pocus, (I thought on a first reading it that he was excluding the signing of the document from the meaning of "Fratze" but Faust includes the gestures with the formal confirmation. They are all one to him). I think this is a parody of Lutheran "justifcation by faith" versus Roman Catholic finding the way to Heaven through good deeds, the devilish contrast here being the Faustian "word" versus Mephistopheles' need for deeds. I would make a small but significant change to my suggestion and that is to replace "gesticulation" with gesture, thereby including the signing of the document in the meaning of "Fratze". I can think of no English word which conveys the double meaning of Fratze as grimace and gesture. My understanding of "farce" is that it is something pitiful, humorous, unacceptable-I don't see that in the word "Fratze" (well perhaps a little of the humorous). To sum up-I agree that I had a too narrow interpretation of "Fratze" but I still don't like "farce" at all, I don't think it conveys anything of the menace and Gothic of the original.