The earth-shattering Reformation of the 1500s started on this premise: what is impossible with men is possible with God.
If we want that Reformation today in America, then we need to take to heart that lesson. If we want the Spirit to be shed abroad in our churches, then this verse must become real in our heads and in our hands.
The previous verses report that the disciples were amazed with Christ's doctrine of salvation: that is was harder for a rich man to enter the kingdom of God than to enter into the eye of a needle. And it was not just any rich man that Christ was talking about, but that very rich ruler who claimed to have obeyed the Ten Commandments his entire life (v.20, 21). Christ immediately went for the spiritual jugular: "you think you've obeyed the Commandments? How about giving up on your self-righteousness and obey me? Give up on your lust for wealth and repent and believe in me (follow me)" (v. 22).
What does this have to do with possibilities and impossibilities? you may astutely ask.
Dear reader, Christ is telling you and me that salvation is impossible outside of God. In that Jewish day and age, this impossibility is concretely displayed through the impossibility of the rich man to enter into heaven. The listeners got the point: "WHO THEN CAN BE SAVED?" (v.26).
And Christ's answer was not: "those who desire it enough," "anyone who wants to," or even "just those who repent and believe" (which is certainly true!). No, He held his ground to bring his listeners to the point of needing a savior. He perceived that to tell his audience at that time to repent and believe was to tell them something they would think was within the reach of everyman. Instead, He walled them up within themselves to see how truly hopeless their sinful situation was.
Salvation is impossible if only man is involved. It is not simply that God calls and man in and of himself says "yes"--no, such a situation is impossible! For man is dead in trespasses and sins (Eph.1:1-3). That is the Bad News. Christ said it was impossible for man to be saved without the possibility-changing, almighty, effectual work of God the Sovereign.That is the Good News!
John summarizes this possibility-changing God as the one who births His own: man cannot birth himself--that is impossible (John 3:4,8), only God can do that.