What are the angels and the archangels
The angels and archangels are according to the gospel spirits administrators: Are not all angels ministering spirits sent to serve those who will inherit salvation?, Hebrews 1:14. That is, they are administrators of the divine wisdom, this administration can consist of contemplation: "angels enjoy a holy and pious contemplation", Summa Theologiae Q 58, A1, or reveal, communicate to men the mysteries of God: "the prophets are instructed by the angels" Summa Theologiae Q 57 A5.
The celestial minds or understandings serve to the perfection of the universe and the divine providence, their study is fundamental in the scholastic theology.
Angel means messenger (Summa Theologiae Q 108, A5), and Thomas Aquinas explains: "The ecclesiastical hierarchy is derived from the celestial hierarchy" Q 106 A 3, that is, exists in the empyrean heaven a hierarchy of celestial spirits, the first hierarchy is: the seraphim, cherubim and thrones; the second hierarchy or principality: dominations, virtues and powers; the third: principalities, archangels and angels.
The first principality is in the old testament, the second and third hierarchy of celestial beings is due to Ephesians 1:21: "God constituted Christ as man over all principality, and power, and virtue, and dominion" (Q. 108, A2); and the Letter to the Colossians: "For him all things where created: things in heaven and on earth, visible and invisible, whether thrones or dominations or principalities, or powers; all things have been created for him", Colossians 1:16.
Thomas Aquinas reminds us: "to the saints is promise equality with the angels" Summa Theologiae Q 62, A 5, that is to say, that by eternal life we are participants in the life of the angels, holiness consist to receive the eternal life . And what is eternal life? Very simple, the same knowledge of God: "this is eternal life that they may know you the only true God", John 17: 3.
This is why we can affirm that the goal of the spiritual life is the neighborhood, the communion with the angels and the celestial spirits.