This is said as further encouragement for Mary to believe. It is important that she believes, for the incarnation is among the great works that God has done, and his works are intended to not only achieve wonderful things, but to be appreciated by his people and a source of praise to his holy name.
The believer has peace in his mind, even though he lacks the power to do what is necessary to secure his eternal future. His hope depends on so many things beyond his ability to accomplish. Our religion is most definitely a supernatural religion. Before we can enjoy the glories of heaven, we must experience the return of Christ, the resurrection from the dead, the removal of sin from our souls, the enlightening of our minds, the removal of the curse, the creation of a new eternal home with God, and so many other things that escape our notice. All of these difficulties are met with the assurance that ‘with God nothing shall be impossible.’