(Go to this verse :: Verse pop-up)
Jesus is prophesying of the giving of the Spirit, which is absolutely essential to the "circumcision of the heart," to "writing God's law on the heart," to enabling us to have a good relationship with God. Notice that He puts conditions on receiving the Spirit, which is a factor that did not appear much in the Old Covenant prophecies about it. But here the time to make the Spirit available is near, so God's Servant - Jesus Christ - tells us what the conditions are to be if we are going to agree to this Covenant.
He says that we have to believe, to come to Him. If we have been called, we have to respond. We have to thirst - to want it desperately - and on top of that, we have to drink. Remember the old cliché, "You can lead a horse to water, but you can't make him drink"? A lot of people are like that: They can be led to the truth, but to get them to take it in and make it a part of them is very difficult indeed.
In addition, the Spirit would not be given until Christ was glorified, that is, until after His death and His resurrection to spirit life.
John W. Ritenbaugh
The Covenants, Grace, and Law (Part 12)