Zakariyya was amazed when his supplication was answered and he was given the good news of a son. He became extremely overjoyed and asked how this child would be born to him, and in what manner he would come. This was particularly amazing because his wife was an old woman who was barren and had not given birth to any children in her entire life. Even Zakariyya himself had become old and advanced in years, his bones had become feeble and thin, and he had no potent semen or vigor for sexual intercourse. The Answer of the Angel
(He said:) That is, the angel, in his response to Zakariyya and his was amazement.
("Thus says your Lord: `It is easy for Me..."') Meaning the birth of the son will be from you and from this wife of yours and not from any other (woman).
(easy) Meaning, it is simple and easy for Allah to do. Then he (the angel) mentioned to him that which is more amazing than what he was asking about. The angel said that the Lord said,
(Certainly I have created you before, when you had been nothing!) This is similar to Allah's statement,
(Has there not been over man a period of time, when he was not a thing worth mentioning) 76:1