Matthew 20:22

But Jesus answered, and said
To her two sons,

ye know not what ye ask.
They were ignorant of the nature of Christ's kingdom, which is spiritual, and not of this world: or they would never have asked such a question, or sued for that which will never be enjoyed by any and supposing that Christ's kingdom had been such as they imagined, yet in asking for honours and riches, they might not know what they asked for; they might promise themselves much pleasure and happiness in the enjoyment of them, and yet, if indulged with them, might be disappointed, and find unexpected troubles and uneasiness. It would have been much more proper and seasonable, on hearing of Christ's being mocked, scourged, spit upon, and crucified, if they had put such a question to themselves, Christ here directs to,

are ye able to drink of the cup that I shall drink of, and to be
baptized with the baptism I am baptized with?
meaning his reproaches, sorrows, sufferings, and death; which because of the disagreeableness of them, he compares to a bitter cup of vengeance, wrath, fury, and indignation; and because they were appointed to him, and allotted for him, they were his portion, therefore he expresses them by a "cup"; and because they were so many and great, of such an overwhelming nature, that he seemed to be plunged into them, and covered with them, therefore he likens them to a "baptism" and which the ordinance of water baptism, performed by immersion, is a lively representation of. Now Christ suggests to these disciples, that instead of indulging their ambitious desires of worldly grandeur, that they would do well to consider what a bitter cup he had to drink of, and what a sea of sorrows and sufferings he was about to be plunged into, and drenched in; and whether they could think of enduring anything of the like kind, for his sake, which was most likely to be in a short time, what they would be called unto, and not to honours, ease, and pleasure; and what they must be sure, more or less, to undergo, before they entered the everlasting kingdom of glory:

they say unto him, we are able;
not considering the nature of these sufferings, and their own weakness; but partly through ignorance of themselves, and a vain confidence which possessed them; and chiefly through a vehement desire of the places in his kingdom, they asked for, and which they thought drinking his cup, and being baptized with his baptism, were the condition, and the means of enjoying; and so rashly affirm their ability, and which includes their willingness to comply herewith.