The symbol of the sword has different means across different cultures. In this painting, however, we look at the symbol of the sword through the lens of Christianity. In Christendom, a sword is a symbol of the Word of God, who is Jesus Christ. We are told in Ephesians 6:17 that the sword of the Spirit is the Word of God; an offensive weapon used against the enemies of God. We are also told in John 1:1, 14 that the Word of God is Jesus Christ. And finally, that through this Word all things we made (See Colossians 1:15-16)
How does this fit into the context of the painting?
In the painting above, we see a sword in a frame, or you could say a window. It is seen floating in the clouds; symbolic of Jesus in the Heavens; with the frame serving as a portal into another world.
When standing before the painting, one feels as though s/he can reach out to grab the sword; thus making this sword accessible to all who gaze upon it. This is true about the Word of God. It is also accessible to all who gaze or keep their minds and hearts focused on Jesus Christ. We have access to the Sword and we can reach out and use the sword, when the enemies of stand or oppose the calling and mandate of God.
For further reading:
Ephesians 6:17 (NKJV)
And take the helmet of salvation, and the sword of the Spirit, which is the word of God;
John 1:1 (NKJV)
The Eternal Word
In the beginning was the Word, and the Word was with God, and the Word was God.
Colossians 1:15-16 (NKJV)
He is the image of the invisible God, the firstborn over all creation. For by Him all things were created that are in heaven and that are on earth, visible and invisible, whether thrones or dominions or principalities or powers. All things were created through Him and for Him.