True, but one would think more so in this case.. The Blood of Christ takes center stage from the Last Supper through to the Crucifixion, those events are the origins of the artifacts.
But that was his pledge when he smeared his blood on the Placard wasn't it? We still don't know what if anything the Placard does. Harry said, now if the bad guys wanted in they'd have to go through Mac. Well, wasn't that the case before he pulled out the Placard? Did it make the bar door a super threshold? It never sent out power vibes like the Spear did. When Murphy asked Harry what it was, he said it was the sign above Christ's head on the Cross, and that it stood for intercession. Looked that word up and it had a lot to do with asking God or some deity for aid. Which for Mac, as an exiled angel playing strictly neutral might not be that easy, unless at last he has decided to take a stand and a side for humanity's sake.
Of course if we just take not intercession, but the verb intercede, it just says 'intervene on behalf of another.' So I'm more on the following argument:
By taking up the placard, Mac is intervening on behalf of the people inside his bar. He intercedes between whatever may come at them and his guests. Hence, he's not taking a side, he's focusing any outside threat on him, whatever he really is, which is more powerful than just pretending to be a guy with a shotgun.
Likewise, in the crucifixion, TWC placed Himself between whatever ailed them (possibly the sins of mankind) and mankind or maybe just between those ills and the Jewish people (what with the inscription, at least in our world, mentioning the Jews). But in that case, through the additional artifacts used, it probably worked more permanently (or maybe TWC is just hiding deep in the NeverNever so that noone can break that intercession).