Albert Barnes on Matthew 20:22

Ye know not what ye ask. You do not know the nature of your request, nor what would be involved in it. You suppose that it would be attended only with honour and happiness if the request was granted; whereas, it would require much suffering and trial.

Are ye able to drink of the cup, etc. To drink of a cup often, in the Scriptures, signifies to be afflicted, or sometimes to be punished, Isa 51:17,22, Ps 75:8. The figure is taken from a feast, where the master of a feast extends a cup to those present. Thus God is represented as extending to his Son a cup filled with a bitter mixture --one causing deep sufferings, Jn 18:11. This was the cup to which he referred.

The baptism that I am baptized with. This is evidently a phrase denoting the same thing. Are ye able to suffer with me---to endure the trials and pains which shall come upon you and me in endeavouring to build up my kingdom? Are you able to be plunged deep in afflictions, to have sorrows cover you like water, and to be sunk beneath calamities as floods, in the work of religion? Afflictions are often expressed by being sunk in the floods, and plunged in the deep waters, Ps 59:2, Is 43:2, Ps 124:4,5, Lam 3:54.

(o) "baptism" Lk 12:50