Angels are spirits, 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 their nature. You ask the name of this nature, it is spirit; you ask its office, it is that of an Angel, which is a messenger.
Saint AugustineNarrow is the mansion of my soul; enlarge Thou it, that Thou mayest enter in. It is ruinous; repair Thou it. It has that within which must offend Thine eyes; I confess and know it. But who shall cleanse it? or to whom should I cry, save Thee? Lord, cleanse me from my secret faults, and spare Thy servant from the power of the enemy. I believe, and therefore do I speak.
Saint Augustine