The meaning is: "Don't mix the sacred with the profane" (see reference below), or "Joke with people but don't mess with the Saints".
Another quotation: "Scherza coi fanti e lascia stare i santi" is a proverb that means, more or less, that you may joke with unimportant people ( fanti, that means simple soldiers) but not with your superiors (santi = saints, intending important people). I think that "fanti" and "santi" are choosen because of the rhyme, not for else.
Here you can find a definition and some more examples, in case you need other religion-related proverbs:
http://docs.google.com/viewer?a=v&q=cache:j6hQAakQoc8J:www.h...

Italian term (edited): scherza coi fanti e lascia stare i santi

Pervasi dal senso dell’onnipotenza non si rendono conto di quanto poco interessano alla gente queste esternazioni personali, anzi infastidiscono memori del detto “scherza coi fanti e lascia stare i santi”.

Pervaded by a feeling of omnipotence, they fail to realise how little people are interested in their personal attitudes and statements, which are, as a matter of fact, rather annoying, since the saying "Don't mix the sacred with the profane" still lingers in people's minds.

http://shortify.com/10960

It's a quote from Puccini's opera Tosca, where at the very beginning of the opera the Sacristan says to the painter Cavaradossi: "Scherza coi fanti e lascia stare i santi" translated variously as "Jest with knaves and leave the saints alone".

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Note added at 19 mins (2010-08-26 17:40:09 GMT)
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Sorry I didn't complete the phrase. I wanted to say that there are a number of translations, but they all carry the same meaning.

Cavaradossi (al Sagrestano)
Dammi i colori!
(Il Sagrestano eseguisce. Cavaradossi dipinge con rapidità e si sofferma spesso a riguardare il proprio lavoro: il Sagrestano va e viene, portando una catinella entro la quale continua a lavare i pennelli.)
(A un tratto Cavaradossi si ristà di dipingere; leva di tasca un medaglione contenente una miniatura e gli occhi suoi vanno dal medaglione al quadro).
Recondita armonia
di bellezze diverse!...
È bruna Floria,
l'ardente amante mia...

Sagrestano
(a mezza voce, come brontolando)
Scherza coi fanti e lascia stare i santi!
(s'allontana per prendere l'acqua onde pulire i pennelli)

English translation (one of many)
CAVARADOSSI
Give me my paints.
The Sacristan does so. Cavaradossi paints rapidly, with frequent pauses to observe his work. The Sacristan comes and goes; he carries a small basin in which he continues his job of washing the brushes.
Suddenly Cavaradossi leaves his painting: from his pocket he takes a medallion with a portrait in miniature, and his eyes travel from the miniature to his own work.

Oh hidden harmony
Of contrasting beauties! Floria
Is dark, my love and passion…

SACRISTAN
to himself
Jest with knaves and neglect the saints …

Non è molto poetico ma questo è il senso.

http://dizionari.kataweb.it/

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Note added at 1 ora (2010-08-26 18:57:49 GMT)
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Ovviamente vale per la frase intera: "scherzare coi fanti e lasciar stare i Santi".

VERY free adaptation ;-)

why not play with words?
All Hallows = All Saints

it would keep the reference to the pumpkin traditionally associated to this religous+pagan event

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Note added at 17 hrs (2010-08-27 10:29:39 GMT)
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p.s. saints referred to in the second part of the proverb (cherza coi fanti, e lascia stare i santi)