Studying Chapter 7 of Jesus the Christ, where Gabriel announces the births of John and Jesus. John is the forerunner to Jesus.
It is totally understandable that Zacharias was incredulous when Gabriel announced Elizabeth's impending conception. This part of the history of John and Jesus teaches me how important faith is. When the Holy Ghost whispers instructions, it isn't ours to scoff at particulars. We must follow the will of God and believe that all things can be done through Him. Although Mary's reaction was similar, hers was mere confusion instead of disbelief. It is fine to question but not ok to let faith falter.