Here is my league deck of Saint Celestine.
It served me well.
Saint Celestine is a tricky warlord to play. It can be very pleasant, but if you lack faith, you're dooomed. (Well, a Saint lacking faith...)
The hidden gem is the Interrogator Acolyte. You throw a 2/2 in the fight by surprise AND it's going to give you back 3 cards! (1 for Saint Celestine ability, 2 for his own ability)
(I tried the Enginseer as well, but I always fail to get any support, so I removed it)
Mystic Warden is cool too. No more need to play them on P1. Just keep them in your hand for the moment your opponent is confident you have no faith to trigger your ability, and boum. For free, and if your opponent must flee, +1 card. Perfect.
Elysian Assault Team is here to keep the pressure. Because Saint Celestine ability alone may not always be enough to win.
Finally, with 3x Ammo Depot + 3x Banner of the Sacred Rose + 2x Agra's Preachings, the deck is a swarming crusade. Tons of units out everywhere and Saint Celestine to give the little "push" to finish the job.
(I have seen a lot of midrange Saint Celestine, in order to ambush bigger stuff I presume, but I don't have any idea how to make a correct midrange Saint Celestine.)
And in case you didn't notice (I didn't, until I listen Slaanesh-Devotee's review) the Heralding Cherubim triggers on Miraculous Intervention. It requires a bit of planning, but to be honest this is the best signature unit ever. It's so good and so enjoyable. "We beseech thee O Saint Celestine!"
4x Heralding Cherubim
1x Captain Markis
1x Sacalleum's Finest (because!)
3x Elysian Assault Team
3x Interrogator Acolyte (synergyyyyy)
2x Sacred Rose Immolator
3x Tallarn Raiders
3x Mystic Warden (synergy again!)
3x Zealous Cantus
1x Devoted Hospitaller
1x Patron Saint
1x Exalted Celestians
3x Tenacious Novice Squad
3x Banner of the Sacred Rose
1x Armour of Saint Katherine
3x Preemptive Barrage
1x Our Last Stand
2x Miraculous Intervention
3x Ammo Depot
2x Inquisitorial Fortress
3x Holy Chapel
2x Agra's Preachings
1x Order of the Crimson Oath