The main idea of the Gettysburg Address is that the dead should be remembered by our recommitment to unity and equality. It communicated largely through Lincoln's use of parallel structure, antithesis, and poetic rhythm. The diction is also noticeable not only for the words Lincoln used but for those he chose not to use.
The speech is often caused a "prose poem" because the rhythm and structure Lincoln used is often found in poetry. The use of anaphora in the lines "we can not dedicate, we can not consecrate, we cannot hallow" create a rhythm that clearly points the audience to what they need to do. Using parallel structure ,Lincoln tells people they must work towards a "government, of the people, by the people and for the people."
Lincoln also uses imagery and antithesis when he compares birth and death but he ingeniously focuses on images associated with birth when he uses the words "brought forth", conceived,"and "new birth of freedom." Finally, his use of active verbs gives the audience hope for the future. He states, "the dead shall not have died in vain" the government shall not perish" to refocus the audience's attention to moving on after the terrible battle. Finally, he binds the audience together by not using the words North/South, Union/Confederacy or slavery. Thus the audience was able to turn away from the very concrete problems of the day and allow themselves to be inspired by Lincoln's words.
One of the brilliant structural aspects of the speech is the way it traverses time. It starts 87 years ago with the opening:
Four score and seven years ago our fathers brought forth...
Then the middle section of the speech focuses on the current crisis:
Now we are engaged in a great civil war.... We have come to dedicate a portion of that field
And finally the speech concludes with the emotive description of the larger purpose that lies in the future:
dedicated to the great task remaining before us... that government of the people... shall not perish from the earth.
By speaking of the past, present, and future, Lincoln is able to evoke the very substance of human existence.