The word [firstfruits] is not simply an indication of temporal priority. Rather, it brings into view Christ's resurrection as the "firstfruits" of the resurrection-harvest, the initial portion of the whole. His resurrection is the representative beginning of the resurrection of believers. In other words, the term seems deliberately chosen to make evident the organic connection between the two resurrections. In the context, Paul's "thesis" over and against his opponents is that the resurrection of Jesus has the bodily resurrection of "those that sleep" as its necessary consequence. His resurrection is not simply a guarantee; it is a pledge in the sense that it is the actual beginning of the general event. In fact, on the basis of this verse it can be said that Paul views the two resurrections not so much as two events but as two episodes of the same event.
From Resurrection and Redemption: A Study in Paul's Soteriology by Richard B. Gaffin Jnr, (2nd Editon, 1987, P&R, p. 34 & 35)