This verse begins a section introducing Paul's admonition to hold fast what has been entrusted to our care, which he calls "the testimony of our Lord."
What is a testimony? Most commonly, it is used when a person is called upon to give an account of what he witnessed. This, however, is a narrow usage.
In a broader application, Webster says that it means "firsthand authentication of a fact," which is what one is called upon to do in a court trial, to verify a fact. A trial lawyer may ask, "Did you know this person before such and such a date?" The witness then authenticates whether or not this fact is true. Testimony also means "evidence." The lawyer asks, "What did you see?" And then the witness presents his evidence.
But it can also mean "a solemn declaration, an open acknowledgment." This is closer to what Jesus Christ did. He gave an open acknowledgment, a solemn declaration, of a message that He left with mankind. That was the testimony of our Lord, the message of the Messenger. The church knows it as the gospel of the Kingdom of God.
To turn the last clause of this verse into plain English, God began His purpose before time! Not only is the fulfillment of the gospel yet future, its beginning stretches all the way back before time began as human beings look at it. At some point in the distant past before mankind, God's purpose began moving toward completion.
If the gospel began before time, and if it is the essence of future events, then we can logically conclude that God's purpose is not completed! Completion of the purpose, of the good news, is still future. Whatever lies in the future is the goal toward which the purpose is moving, and that goal is the good news. Of course, there will be wonderful and encouraging accomplishments along the way. We could call them benchmarks. Although alone they are good news, it is the culmination of them that is the good news.