"How will I know this for certain?" Zechariah asked and God called his question unbelief. Mary asked "How will this be?" and God said she believed. So why was Zechariah’s question not acceptable and Mary’s was?
Know
Lavish Love, Pt. 2
Do you have a hard time knowing and believing that God loves you? The verb "to know" in Scripture means to lay hold of so as to make one's own, to obtain, to take into oneself, to appropriate, to take possession of. Knowing implies realizing something, being sure of it and feeling it.
Pondered Treasures 