I'll certainly believe again if they win Game 6
I do think Game 6 is essentially going to be a Game 7 for both teams. PHI has looked great, but I think even they know they need to wrap this up in 6 at their homecourt and not risk having to go back to Boston for a Game 7. If Boston loses, of course series is over. But if Boston wins Game 6, I think they firmly have the advantage for Game 7.
Even though TD Garden probably is cursed, the C's do very well at home in Game 7s. Won Game 7 against MIL at home last season. I know we dread 2018 ECF Game 7 against Lebron's Cavs, but since 2017 the C's have gone 3-1 in home Game 7s, and I guess 4-1 if you count 2020 Bubble vs. Toronto. Also had the big Game 7 road win last season against Miami. I don't think a Game 7 would scare them. Funny enough, it's the games in-between where they seem to play worse, but they show up in the "backs against the wall" games almost always (the elimination games)