5 Now seeing that heretics cannot deny these things because they are so clearly stated and understood, they nevertheless pervert them by the most foolish and wicked lies so as afterwards to deny them. For the words of Christ, I and the Father are one, they endeavour to refer to a mere concord of unanimity, so that there may be in them a unity of will not of nature, that is, that they may be one not by essence of being, but by identity of will. And they apply to the support of their case the passage in the Acts of the Apostles, Now of the multitude of them that believed the heart and soul were one, in order to prove that a diversity of souls and hearts may be united into one heart and soul through a mere conformity of will.
Or else they cite those words to the Corinthians, Now he that plants and he that waters are one, to show that, since They are one in Their work for our salvation, and in the revelation of one mystery, Their unity is an unity of wills. Or again, they quote the prayer of our Lord for the salvation of the nations who should believe in Him: Neither for these only do I pray, but for them also that shall believe in Me through their Word; that they all may be one; even as Thou, Father, art in Me, and I in You, that they also may be in Us, to show that since men cannot, so to speak, be fused back into God or themselves coalesce into one undistinguished mass, this oneness must arise from unity of will, while all perform actions pleasing to God, and unite one with another in the harmonious accord of their thoughts, and that thus it is not nature which makes them one, but will.
Source: On the Trinity (New Advent)