In our pre-mortal life, we existed as intelligences and progressed as far as we could without physical bodies. To continue progressing and become like our Heavenly Father, we needed to experience mortality and gain physical bodies through the Plan of Salvation. Jesus Christ volunteered to be our Savior and die for our sins so we could overcome death. However, Lucifer wanted God's power and glory for himself, which led to a war in heaven where Lucifer and his followers were cast out. God then created the earth so we could continue our progression by being tested through mortal life.