The danger is that we begin looking for a way out of the trial rather than appreciating what God is doing with the trial. James therefore gives a command to allow endurance have its perfect work. The producing of endurance in the life is not the end but part of what God uses to accomplish His purpose in the life of the believer. This is described in three ways in the last part of the verse. The idea is that of maturity, being all that we should be as spiritual adults in Christ.
Every necessary part is present and functioning.
God is in the process of making us everything we should be as His children. He intends for us to be mature, functioning in every part of our being exactly as He intends. For this to be accomplished, trials are necessary. They build in us an endurance, a steadfastness under pressure which will develop us as mature men in Christ. All quotations used by permission.