How do we know?
She deliberately chose to act in faith.
She went and told Joseph.
She couldn’t prove she was pregnant.
She couldn’t prove that the baby wasn’t because of an affair.
She couldn’t even convince her fiancé until an angel showed up.
But her belief wasn’t unfounded.
She had it on pretty substantial authority.
She knew that when angels show up, you listen,
Even so, there is no way Mary could have been absolutely certain.
Yet, she was willing to act.
Maybe that’s all that’s required.
38 Then Mary said, “Here am I, the servant of the Lord; let it be with me according to your word.” Then the angel departed from her. – Luke 1:38