Beacon of Faith lets Seraphim "add 1 to their Shield of Faith invulnerable saves". It is a little unclear whether this is intended to be a modifier to the saving throw or an improvement to their actual save. Seraphim actually force you to "re-roll failed Shield of Faith invulnerable saves" -- you don't get a choice. This is very strange in both cases. Usually we're told to modify rolls or saving throws rather than saves, and usually we're given the option to re-roll failed rolls.
If Celestine is giving them a modifier to their roll, then I guess the way this works is that you first get a 6+. A roll of 5 is a failure, before modifiers, and so you would have to re-roll it. And then you would save on a 5+ the second time.
If Celestine is improving their save by 1, then I think it's a 5+ both times. This wouldn't be a modifier to a roll.
Generally the rules seem consistent about explicitly telling you to modify a roll when that's what they mean. You never add or subtract one from "hits" but from "hit rolls". A unit in cover can "add 1 to its models' saving throws" rather than to its "saves". Thousand Sons HQs allow re-rolling of "invulnerable saving throws of 1". Rubric Marines "add 1 to the saving throws..." The Tzeentch Ephemeral Form ability likewise adds to saving throws.
Meanwhile there are very few abilities that modify characteristics which you test against. The Necron HQ Nemesor Zahndrekh can take a unit and "improve their Ballistic Skill by 1". Obviously what's weird about this for Celestine is that "adding 1" to the Seraphim's invulnerable save doesn't seem to make sense, since lower is better.
However, note that the Dark Eldar combat drug Serpentin has the effect: "+1 to Weapon Skill (i.e. WS 3+ becomes WS 2+)", which clearly suggests that we should interpret addition to test characteristics as improving them despite this seeming backwards.
Given Serpentin, I'm comfortable saying that what Celestine does is make the Seraphim's invulnerable save 5+, instead of adding 1 to their saving throws. I think this is probably not a modifier of the sort that would require you to re-roll a 5, but this is not 100% clear.
I am also very confident that almost nobody who thought about these rules at any point, inside or outside GW, even considered the 6++/5++ reading.