In my Father's house are many mansions: if [it were] not [so], (a) I would have told you. I go to (b) prepare a place for you.
(a) That is, if it were not as I am telling you, that is, unless there was room enough not only for me, but also for you in my Father's house, I would not deceive you in this way with a vain hope, but I would have plainly told you so.
(b) This whole speech is an allegory, by which the Lord comforts his own, declaring to them his departure into heaven; and he departs not to reign there alone, but to go before and prepare a place for them.