Now I must explain what "firstfruit" means. Under the Law of Moses the farmer would take a handful of grain and give it to the priest who offered it as "firstfruit." If it was good it meant that there would be a great harvest. Christ being the firstfruit means there would be a great harvest of souls for Heaven. Who are the harvesters or the reapers? They are the Christians who bring in precious souls for the kingdom of God.
Something can be said about this statement that Christ's enemies will be put under His feet. God will do that someday, as it is written more than once in the Old and New Testaments. The last enemy destroyed was death. For us the cross is where we should have died, but Jesus took that for us.
The beauty of this is that Jesus came here and though He was creator, He became a man, subject to God. Now when the Kingdom is fulfilled, He takes it and gives it back to God that God may be all in all.