John Chrysostom

At least, when the disciples were troubled, He said, "With men this is impossible; but with God all things are possible." Therefore, having first "beheld them, He said unto them, The things which are impossible with men, are possible with God." For with a mild and meek look, having soothed their shuddering mind, and having put an end to their distress (for this the evangelist signified by saying, "He beheld them"), then by His words also He relieves them, bringing before them God's power, and so making them feel confidence. …

Homily on the Gospel of Matthew 63 ↗

Thomas Aquinas

And Jesus beholding, said to them: with men this is impossible etc. Here he responds to their astonishment, saying: with men this is impossible, but with God all things are possible. But what is it that he says? For it seems that free will perishes if it is impossible with men. It is true that man has it of himself that he can sin; but to rise again and to do works of salvation, this he does not have of himself without the help of God's grace: for it is God himself who can do these things. …

Commentary on Matthew ↗

Remigius of Rheims

This must not be so understood as though it were possible for God to cause that the rich, the covetous, the avaricious, and the proud should enter into the kingdom of heaven; but to cause him to be converted, and so enter.

Catena Aurea by Aquinas ↗