“Therefore I beseech and entreat you, St Michael the Archangel, who knows those of the accepted souls to be received, find my soul worthy when it leaves my body and free it from the power of the enemy, so that it avoids the gates of hell and the ways of shadows and that the lion and dragon who usually receive souls in hell and lead them to eternal torment.” [*]
– from Oxford Bodleian Library MS Douce 296, fol. 122v. Cited in and translated by Kathleen Openshaw, “The Battle between Christ and Satan in the Tiberius Psalter,” Journal of the Warburg and Courtauld Institutes 52:14-33.
ETA: inkysaurus wonders, “Can you shed a little light for us non-Catholics on how St. Michael is both a saint and an archangel?”
That’s a good question, since we tend to think of saints as formerly living humans who led exemplary lives and performed miracles and now hang out in heaven doing various jobs and interceding for the living who petition them. I guess that’s how people think of them anyway. But according to the Roman Catholic church, whose stance is explained at the Catholic Encyclopedia, a saint is basically someone who’s in heaven. Or, to put it another way, if you’re in heaven you’re a saint. They write,
The communion of saints is the spiritual solidarity which binds together the faithful on earth, the souls in purgatory, and the saints in heaven in the organic unity of the same mystical body under Christ its head, and in a constant interchange of supernatural offices. The participants in that solidarity are called saints by reason of their destination and of their partaking of the fruits of the Redemption…
St. Thomas teaches (III:8:4) that the angels, though not redeemed, enter the communion of saints because they come under Christ’s power and receive of His gratia capitis. The solidarity itself implies a variety of inter-relations: within the Church Militant, not only the participation in the same faith, sacraments, and government, but also a mutual exchange of examples, prayers, merits, and satisfactions; between the Church on earth on the one hand, and purgatory and heaven on the other, suffrages, invocation, intercession, veneration.
And they summarize Augustine’s take:
In the transcendent view of the Church taken by the latter (Enchiridion 66) the communion of saints, though never so called by him, is a necessity; to the Civitas Dei must needs correspond the unitas caritatis (De unitate eccl., ii), which embraces in an effective union the saints and angels in heaven (Enarration on Psalm 36, nos. 3-4), the just on earth (On Baptism III.17), and in a lower degree, the sinners themselves, the putrida membra of the mystic body; only the declared heretics, schismatics, and apostates are excluded from the society, though not from the prayers, of the saints (Serm. cxxxvii).
The Catholic church characterizes the communion of saints as “that reciprocal action of the saints, that corporate circulation of spiritual blessings through the members of the same family, that domesticity and saintly citizenship…”
So basically, I guess the short answer is that St. Michael can be both a saint and an angel because the Roman Catholic definition of a saint is not the same as the commonly accepted definition of a saint.
[*] “Te ergo supplico et deprecer sancte michael archangele qui ad animas accepiendas accepisti postestatem ut animam meam suscipere digneris quando de corpere meo erit egressa et libera eam de potestate inimici ut pertransive possit portas infernorum et vias tenebrarum ut non se deponat leo vel draco qui conseutus est animas in inferno recipere et ad aeterna tormenta perducere.”