Saint Francis de Sales talks about the last will and testament of Jesus on the Cross:
"He still had one thing to add to his last will and testament. But what else was there, you may ask, my dear Sisters?
There exists a certain spiritual delicacy, which he wished to bestow upon his dearest friends, a delicacy which is nothing other than a very singular means of preserving the grace he had acquired and of reaching the highest degree of glory.
Looking therefore with compassionate eyes at his most blessed Mother, who was standing at the foot of the Cross with his beloved disciple, He did not want to give her or obtain grace for her, for she already possessed it very excellently, nor did he want to promise her glory, for she was already assured of it; but he gave her a certain union of heart and tender love for neighbor, that mutual love which is one of the greatest gifts that his goodness bestows on men. “Woman," he said, "behold your son."
O God, what an exchange! She went from having the Son, to having the servant; from having God, to receiving a creature! Nevertheless, she did not refuse, knowing well that in the person of St. John she was accepting as her own all the children of the Cross and that she would be their dear Mother. Our divine Master taught us by this that if we want to share in his testament and in the merits of his Death and Passion, we must love one another with that tender and deep love of the son for his Mother and of the Mother for her son.”
St. Francis de Sales