Q. Why would Jesus be called a “thing” in Luke 1:35?
The problem is related to the version you are using.
Luke 1:35 – KJV
35 And the angel answered and said unto her, The Holy Ghost shall come upon thee, and the power of the Highest shall overshadow thee: therefore also that holy thing which shall be born of thee shall be called the Son of God.
The KJV is a good version of the bible, but not the best available for study today. The more accurate translation is:
Luke 1:35 (NAS95)
35 The angel answered and said to her, “The Holy Spirit will come upon you, and the power of the Most High will overshadow you; and for that reason the holy Child shall be called the Son of God.
The New American Standard (’95 edition) supplies the word “Child” which is not found in the original, but indicated in the verse.
The New International Version is very close to the original in this verse:
Luke 1:35 (NIV)
35 The angel answered, “The Holy Spirit will come upon you, and the power of the Most High will overshadow you. So the holy one to be born will be called the Son of God.
Literally the Greek says “the holy to be born to you.” Adding “one” or “Child” is an acceptable way to convey the sense of the verse in this instance.