God (motif)
Longfellow contrasts the "belfries of all Christendom" with the "cannon[s] thunder[ing] in the South," implicitly excluding the South from the realm of Christendom, due to the sin of slavery. At the end of the poem as he is regaining his optimism he says that the bells proclaim God is awake and will punish wrongdoers. This evocation of God asserts first how high the stakes are and also how the North is in the right.
Bells (symbol)
The bells are more than just instruments: they symbolize peace, goodwill, victory, triumph, and virtue. Their final ringing out at the end over the sounds of the cannons symbolizes how the North will eventually prevail. They almost seem to be heavenly, suggesting angels and their instruments.
Natural Disasters (motif)
The South is identified with natural disasters such as thunder and earthquakes. The repeated evocation of terrifying and dangerous natural events allows us to see the war in the same way: as a disruption, a disaster. The war certainly is a rupture, a schism much like an earthquake.
