Adam Clarke on Luke 1:45

Blessed is she that believed; for there shall be, etc. - Or, Blessed is she who hath believed that there shall be, etc. This I believe to be the proper arrangement of the passage, and is thus noticed in the marginal reading. Faith is here represented as the foundation of true happiness, because it receives the fulfillment of God's promises. Whatever God has promised, he intends to perform. We should believe whatever he has spoken - his own authority is a sufficient reason why we should believe. Let us only be convinced that God has given the promise, and then implicit faith becomes an indispensable duty: in this case not to believe implicitly would be absurd and unreasonable - God will perform his promise, for He cannot lie.