St. Catherine of Siena experiences her mystical marriage: The Virgin Mary holds her hand out to Christ, who places a ring on her finger. King David's harp provides the music for the scene, witnessed by other Saints...and bunnies.
Medieval Manuscripts Add. 34294,f.204v
Sforza hours, c.1520

Did you know that Fra Angelico illuminated manuscripts? He created this large initial P[rotexisti] in a Gradual, a depiction of St. Peter of Verona, the first martyr of the Dominican Order (†1252). Dying, he writes with his blood: Credo in unum Deum.
Firenze, Museo di San Marco

Depiction of the Trinity as a three-faced figure (banned by Pope Urban VIII in 1628); dogmatic scheme illustrating the identity of Father, Son and Spirit with the divine essence and their difference from one another.
Westmalle Abbey, Groot antifonarium van Vorst, vol. 4, c. 1500

Impressive full page Initial (S) from a 13th c. Psalter showing a man riding a griffin
(Beginning of Ps 68: Salvum me fac Deus...)
Getty Ms. Ludwig VIII 2, f. 77r
Würzburg (Germany), 1240/50

'Beatissimo Papae Damaso Hieronimus' (Beginning of a letter of St. Jerome)
One can only admire the wonderful elegance of this Ottonian initial, produced for a gospel book around the year 1000.
Manchester John Rylands Research Institute and Library Cod. 98, f. 1v