This He says not on account of any inequality between the Word of God and the Holy Ghost, but because the presence of the Son of man amongst them would impede the coming of the latter. For the Holy Ghost did not humble Himself as did the Son, by taking upon Him the form of a servant. It was necessary therefore that the form of the servant should he removed from their eyes; for so long as they looked upon that, they thought that Christ was no more than what they saw Him to be. So it follows: But if I depart, I will send Him unto you.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: i. de Trin. c. 9.