When Jesus began to perform His ministry, the
Holy Spirit began to testify to the name of Jesus, and the name of Jehovah was no longer spoken of; instead, the Holy Spirit undertook the new work principally under the name of Jesus. The testimony of those who believed in Him was borne for
Jesus Christ, and the work they did was also for Jesus Christ. The conclusion of the Old Testament Age of Law meant that the work principally conducted under the name of Jehovah had come to an end. Henceforth, the name of God was no longer Jehovah; instead He was called Jesus, and from here on the Holy Spirit began the work principally under the name of Jesus. … Why is it that Jehovah and Jesus are one, yet They are called by different names in different ages? Is it not because the ages of Their work are different? Could a single name represent God in His entirety? This being so, God must be called by a different name in a different age, and must use the name to change the age and to represent the age. For no one name can fully represent God Himself, and each name is able only to represent the temporal aspect of God’s disposition in a given age; all it needs to do is to represent His work. Therefore, God can choose whatever name befits His disposition to represent the entire age. Regardless of whether it is the age of Jehovah, or the age of Jesus, each age is represented by a name.
from “The Vision of God’s Work (3)” in The Word Appears in the Flesh