The core idea of "Heaven's Foray" is to utilise low-cost creatures, sorceries and enchantments to stall the opponent through disabling permanents and unsummoning while scrying and drawing cards, until you have formed a credible line of defence and counter-attack that will be difficult to block. A very strong mana base helps control the game's tempo.
Cheap flying creatures that reduce your mana costs or summon more of their kind enable you for a flexible early game offence or defence. Heralds of War further help dramatically reduce the costs of your costly but heavy-hitting Angels while dishing out increasing amounts of damage. With five or more mana, the heavier-hitting Serra Angel and Avacyn, Angel of Hope, and Akroma's Memorial can helps settle matters in late game.
All of this deck's creatures have flying, thus giving your opponents a harder time defending, and especially when supported by the deck's control-oriented enchantments and spells such as Negate or Faith's Fetters. Finally, cards like Divination, Foresee, Ring of Three Wishes, and Defy Death can help refill your hand with the cards you are looking for.
"Heaven's Foray" is built for Vintage, Legacy and Modern games. Playing Mask of the Mimic limits it to Vintage and Legacy only.
Your two most vital creatures are the Stormscape Familiar, which reduces the cost of all white spells by 1, and the Herald of War, which reduces the cost of angels by 1 for each counter on her. The two cards combined can offer dramatic reductions to the deck's costs, even for the ultra-expensive Avacyn, Angel of Hope - protect them at all costs.
Ideally you will want to start with at least two lands, including the potential for at least one white and one blue mana. Azorius Guildgates and Azorius Chanceries make this easier, with the latter four also boosting your middle-game mana significantly despite the early-game setback they might provide. At 4 or 5 mana, you can effectively play the entire deck, discounts not taken into account.
Early in play you will want to get a Stormscape Familiar followed by Squadron Hawks as early as possible, depleting your library of all remaining Squadron Hawks in the process. You can then use cards such as Divination and Foresee to keep your hand full and keep drawing the cards you need. Skymark Rocs can help eliminate annoying threats such as Elvish Pipers, Guttersnipes and the likes, or at least delay their applicability. Keep your Supreme Verdict to clear the board when the opponent has a dramatic advantage that can cost you the game, though you can use it more liberally if Avacyn, Angel of Hope has seen the battlefield under your control. Lastly, Negate can help remove non-creature spells that will cause you trouble.
Assuming you play with Mask of the Mimic (sideboard): As soon as you have a big flier such as Serra Angel or Herald of War in the battlefield (or if your opponent has one already and you don't), you will want to use a Mask of the Mimic to sacrifice a Squadron Hawk and obtain either of the aforementioned angels for the measly cost of one blue mana, even at the end of an opponent's turn. From that point on, use can your squadron of big angels to rain some serious pain on your opponents.
Finding Avacyn, Angel of Hope and Akroma's Memorial should be relatively easy with the use of Foresee, Divination, and possibly a Ring of Three Wishes. These two cards can dramatically alter the course of a game, as they can give all your creatures (and in the case of Avacyn, other permanents) a terrible array of new abilities: indestructible, trample, haste, first strike, vigilance, protection from black, and protection from red.
You can use cards like Faith's Fetters and Seraph Sanctuary to obtain some life, especially if you combine the latter with an Azorius Chancery. The card allows you to return the a land to your hand, thus reusing Seraph Sanctuary for more life. Emancipation Angels (sideboard) in particular also allow you reuse your Faith's Fetters on new targets. Elixirs of Immortality can also give some life while returning your graveyard to your library (thus allowing you to use a dead or discarded important card, such as a countered Akroma's Memorial or a slain Herald of War), especially when combined with a Ring of Three Wishes.
One particular possible early-game card combination is possible if you start your hand with at least two lands (one of which must be an Azorius Chancery), Avacyn, and Defy Death. You can willingly use this card combination to discard Avacyn at turn 2 for having too many cards at hand, and then revive her as soon as you have 5 mana (or less, if you control Familiars), bringing her back as a 10/10 Flying Vigilance Indestructible angel of true despair as early as turn 4 if you get the right cards!