To fulfill means to establish
I have not come to destroy, but to fulfill. Some have thought our Lord means, I have come to fulfill this by my entire and perfect obedience to it. And it cannot be doubted that he did, in this sense, fulfill every part of it. But this does not appear to be what he intends here, being foreign to the scope of his present discourse. Without question his meaning in this place, consistently with all that goes before and follows after, is this. I have come to establish it in its fullness, in spite of all the glosses of men. I have come to place in a full and clear view whatever was dark or obscure in it. I have come to declare the true and full import of every part of it, to show the length and breadth (Ephesians 3:18), the entire extent of every commandment contained in it, and the height and depth, the inconceivable purity and spirituality of it in all its branches.
1771 text
3. I am not come to destroy but to fulfil. Some have conceived our Lord to mean, I am come to fulfil this, by my entire and perfect obedience to it. And it cannot be doubted but he did, in this sense, fulfil every part of it. But this does not appear to be what he intends here, being foreign to the scope of his present discourse. Without question his meaning in this place is, (consistently with all that goes before and follows after) I am come to establish it in its fulness, in spite of all the glosses of men. I am come to place in a full and clear view, whatsoever was dark or obscure therein. I am come to declare the true and full import of every part of it: to shew the length and breadth, the entire extent of every commandment contained therein: and the height and depth, the inconceivable purity and spirituality of it in all its branches.