Here are some passages from St. Thomas Catena Aurea on this question:
Pseudo-Chrys., Vict. Ant. e Cat. in Marc.: He spat indeed, and put His hand [p. 155] upon the blind man, because He wished to shew that wonderful are the effects of the Divine word added to action; for the hand is the symbol of working, but the spittle, of the word proceeding out of the mouth. Again He asked him whether he could see any thing, which He had not done in the case of any whom He had healed, thus shewing that by the weak faith of those who brought him, and of the blind man himself, his eyes could not altogether be opened. Wherefore there follows: “And He looked up, and said, I see men as trees walking;” because he was still under the influence of unfaithfulness, he said that he saw men obscurely.
From the commencement, however, of the return of his senses, He leads him to apprehend things by faith, and thus makes him see perfectly; wherefore it goes on, “After that, He put His hands again upon his eyes, and he began to see,” and afterwards he adds, “And he was restored, and saw all things clearly,” that is, being perfectly healed in his senses and his intellect.
Theophylact: But the reason why he did not see at once perfectly, but in part, was, that he had not perfect faith; for healing is bestowed in proportion to faith.
Bede: Or else, putting spittle into the eyes of the blind man, He lays His hands upon him that he may see, because He has wiped away the blindness of the human race both by invisible gifts, and by the Sacrament of His assumed humanity; for the spittle, proceeding from the Head, points out the grace of the Holy Ghost. But though by one word He could cure the man wholly and all at once, still He cures him by degrees, that He may shew the greatness of the blindness of man, which can hardly, and only as it were step by step, be restored to light; and He exhibits to us His grace, by which He furthers each step towards perfection. Again, whoever is weighed down by a blindness of such long continuance, that he is unable to distinguish between good and evil, sees as it were men like trees walking, because he sees the deeds of the multitude without the light of discretion.
Pseudo-Jerome: Or else, he sees men as trees, because he thinks all men higher than himself. But He put His hands again upon his eyes, that he might see all things clearly, that is, understand invisible things by visible, and with the eye of a pure mind contemplate, what the eye hath not seen, the glorious state of his own soul after the rust of sin. He sent him to his home, that is, to his heart; that he might see in himself things which he had not seen before; for a man despairing of salvation does not think that he can do at all what, when enlightened, he can easily accomplish.
