This theme is built mostly from Innistrad Midnight Hunt and Innistrad Crimson Vow. Even though Lunarch Veteran is not a Spirit, it turns into one, and the same is true for some of the other creatures useful for this deck. Also, Lunarch Veteran's lifegain triggers so much life gain that it can boost many other cards, and turbocharge pretty much any deck.
Voice of the Blessed is by far the strongest card in the Spirits deck, and it gets really big on lifegain triggers.
The theme has two legendary creatures that can be used for casual Brawl and Commander builds.
Spirits have a lord: Patrician Geist.
Mirrorhaul Mimic can be used to generate additional Spirits.
Behold the Unspeakable can generate a Spirit token.
