36 But even now that we have proved what was the faith of the Apostle, the heretics think to meet it by the text, My soul is sorrowful even unto death. These words, they say, prove the consciousness of natural infirmity which made Christ begin to be sorrowful. Now, first, I appeal to common intelligence: what do we mean by sorrowful unto death? It cannot signify the same as 'to be sorrowful because of death:' for where there is sorrow because of death, it is the death that is the cause of the sadness. But a sadness even to death implies that death is the finish, not the cause, of the sadness. If then He was sorrowful even to death, not because of death, we must enquire, whence came His sadness? He was sorrowful, not for a certain time, or for a period which human ignorance could not determine, but even unto death. So far from His sadness being caused by His death, it was removed by it.
Source: On the Trinity (New Advent)