Part 1:
"The Angels are spirits," says Saint Augustine, "but it is not because they are spirits that they are Angels. They become Angels when they are sent, for the name Angel refers to their office not to their nature. You ask the name of this nature, it is spirit; you ask its office, it is that of an Angel, [i.e., a messenger]. In as far as he exists, an Angel is a spirit; in as far as he acts, he is an Angel." [ Serm. in Ps. 103, I, 15] The word "angel," comes from a Greek word meaning "messenger." In the Scriptures of the Old Testament, the most frequently used name to designate the Angels is mal'akh, which means, messenger or legate.

This generic name "angel" does not reveal anything about the real nature of those celestial beings besides the fact that they are occasionally sent on a mission as messengers or legates of God to men. Because only on such occasions, and in such a quality, they make themselves visible to men, they have been given the name of messengers from the most common duty and office they fulfill towards God's children here on earth. "And to the Angels indeed he saith: 'He that maketh his Angels spirits, and his ministers a flame of fire.' " [ Hebr. 1: 7. In this passage Saint Paul quotes verse 4 of Psalm 103, the same verse Saint Augustine commented on in the above quotation. ]

The office of being a messenger, an Angel, is neither the most important nor the most common among the duties of the celestial spirits in the court of Heaven; it alone does not offer enough ground for speculation on their true nature and operation.

Heaven is the true country of the good Angels: "Their Angels [of the little ones] in Heaven always see the face of My Father Who is in Heaven." [Matt. 18: 10] Even while engaged here on earth as guardians of the little children, they remain the blessed comprehensors, enjoying the vision of God, "the face of my Father." They are by grace the happy citizens of the Heavenly Jerusalem from the beginning.

"Let us remember," writes Saint Bernard, "that the citizens of that country are spirits, mighty, glorious, blessed, distinct personalities, of graduated rank, occupying the order given them from the beginning, perfect of their kind . . . endowed with immortality, passionless, not so created, but so made-----that is, through grace, not by nature; being of pure mind, benignant affections, religious and devout; of unblemished morality; inseparably one in heart and mind, blessed with unbroken peace, God's edifice dedicated to the Divine praises and service. All this we ascertain by reading, and hold by faith." [De Consideratione, Lib. V, cap. 4.]

All this is really what we gather and ascertain by reading the sources, Scripture and Tradition, regarding the nature, character, and blessed condition of the Angels. All the qualities of the Angelic spirits listed here by Saint Bernard are most beautiful and they are theologically correct. It is Catholic doctrine [D. 428, 1783] that the Angels are pure spirits, incorporeal substances, free and independent from any material body, ethereal or otherwise.

THE CREATURES MOST LIKE GOD, the Angels, show forth best the goodness, the majesty, the glory of God; these are His most perfect images, and so the ones to be multiplied with Divine extravagance. Heaven and earth are indeed full of His glory. Because the Angels are bodiless creatures, pure spirits, it is too often concluded that they are supernatural beings; they are not, God is the only supernatural being. The Angels are natural beings, they belong in, and, indeed, dominate our world. They are creatures as natural as oaks, or sunsets, or birds, or men. To call them supernatural because they are not like ourselves is a part of that provincial pride by which a man puts human nature at the peak of the universe, primarily because he himself is a man.

To PRETEND THEY DO NOT EXIST because we do not see them is like pretending that we never sleep because we have never caught ourselves asleep. There would be much more sense in the Angels exiling us from the world of nature on the basis of a majority vote. We have no monopoly on nature; not even on free will and intellectual knowledge in nature; we have big brothers far outstripping our puny powers, yet nonetheless brothers, a part and parcel of the created world that is so truly ours.

SEEING OURSELVES from the plant or animal level, we can with reason marvel at the nobility of men; if the animals were capable of such things, they would see us as godlike creatures. Looking up at the Angels from our level, we promptly shrink to our proper proportions: of all the created world; we have the least, the most earthbound, the feeblest of all created intelligence and love. Lest that be too humiliating, we can reflect that somewhat the same is true of the Angels: seen from our level, they are creatures so wondrous as to make men doubt their very existence; but seen from the heights of God they are so inadequate an image of His splendor as to be insignificant in comparison with the Infinite.

Father O' Sullivan in his work, ALL ABOUT ANGELS, tells us:  [We quote briefly so as to not violate copyright law.]

"The Angels are pure spirits, the mighty Princes of Heaven who stand before God, gazing on His unveiled presence. They are burning fires of love, filled to overflowing with the plenitude of happiness.

"The Angels are the perfect images of God, mirrors of His Divine perfections, reflecting His love, His beauty, His Holiness, His Power, all His Divine attributes and perfections, but each Angel in his own special way.

"No two Angels are alike, no two are equal. God's perfections are infinite, and the countless millions of Angels reflect these perfections in a divinely marvelous way. No two men, no two women, are identically alike, but the difference between them is relatively slight, whereas the difference between two Angels is vast, complete. Every Angel is specifically different from the other as one species differs from another. All the millions of men and women who people the world, all those who have ever lived or ever will live are of one and the same species, but each Angel is a species all in himself .

"The Angels are unspeakably lovely, they have no shadow of imperfection, no defects. Nothing on this earth can possibly give us an idea of their resplendent glory. No painter, no poet, no artist ever conceived anything like them. They are living replicas of God's beauty.

"Fra Angelico's pictures of the Angels excel those of any other artist so that Michelangelo exclaimed on seeing them: "Angelico must have seen these Angels in Heaven; otherwise, he could never have painted them as he has done." But even Angelico's pictures do not give us the faintest idea of the real Angels.

"St. Bridget, who was favored by God with heavenly visions, tells us that were we to see an Angel in all his beauty, we should be so ravished with delight at the sight of him that we should die of love.

"St. Frances of Rome was favored by the constant vision of her Angel. She says that were an Angel to appear in all his splendor, the light of the sun and moon and stars would become dim in comparison.

"After Our Lord's Resurrection, we read how an Angel descended from Heaven and rolled back the stone that had closed the holy sepulchre. The Sacred Scriptures say that the countenance of the Angel was like lightning and his raiment white as snow. His appearance was so full of majesty that the soldiers whom Christ's enemies had placed to guard the tomb were terrified and dared not to look on him, but fell to the ground as if dead.

"When, therefore, the Angels appear to men, they take a human form so as not to over-awe or confound those who look on them.

"What must be the ravishing beauty of the Heavenly Jerusalem, where these countless millions of glorious Angels are not only resplendently beautiful but all differ from each other so that the perfections of one are completely different from those of the other.

"The Angels reflect God's goodness and sweetness no less wonderfully. They reflect all His Divine virtues and perfections, they enjoy the vision of God's infinite love, and they love Him back with all the mighty strength and intensity of their natures.

"Their happiness is perfect, for they receive of the ocean of God's happiness in a way that no human mind can fathom. The happiness that an Angel enjoys in one moment is so deep, complete and all-satisfying that it exceeds all the happiness that a mortal man could enjoy in a thousand years."