Because it soars upward, the eagle is a symbol of the resurrection or ascension of Christ. By extension, the eagle symbolizes baptized Christians, who have symbolically died and risen with Christ.
Especially when the eagle has a halo (as in the image above), it is the symbol of John the Evangelist. The eagle represents John because of his lofty and “soaring” gospel (it is much more theological in nature than the other three). All four gospels have a mascot associated with them.