Likutei Moharan 53
Abraham and Sarah were the strongest in converting people, because they didn’t physically have children yet. They realized, ‘Why don’t we have children yet? Because our minds aren’t ready. How do we make our mind ready physically? By having a lot of children spiritually.’ It’s a gevalt.
Therefore, Rebbe Nachman says that at the moment when Abraham reached the ‘highest quantity of converting,’ then he had Yitzchak.
Tzadikim are running after people to talk to them about G-d because they want their minds to reach the utmost level of maturity. This is the deepest depth there is. And, therefore, the old tzadikim are the ones with the ability to bless people who have no children that they should have children, because the tzadikim have the key for children in their hands: because they have so many children spiritually, so they are on the level that they are able to give others maturity of the mind. It’s very deep.
That means, if a tzadik blesses you to have children, he doesn’t just bless you that you should have children, he blesses you that your mind should be on the level that it becomes real. What does it mean to be mature, to have this holy mind of having children? It means that your mind and G-d’s mind become one. Because your mind, the human mind, can know one thing and yet do something else. But G-d’s mind is that it becomes real.