The fullness of God is found in Yeshua (John 1:16, Colossians 2:9); and passed on to mankind through Yeshua until we become the fullness as well (Ephesians 3:19, 4:13). The word for "fullness" in these passages in Greek is "Pleiroma."
Yeshua taught a parable that the kingdom of God comes in stages of development as a plant comes to its "fullness" only at the end of the harvest season.
Mark 4:28 – The earth bears fruit of itself, first the blade, then the ear, then the full grain in the ear.
As the full fruit comes at the end, so the full purpose and goal of the kingdom of God comes at the end. This process takes place across history until the "fullness" of the times and seasons (Ephesians 1:10).
Since Yeshua is both the Head of the Church (Ephesians 1:22) and the King of Israel (John 12:12-13), the process of the kingdom of God coming to its fullness is divided into two halves: one through the Church and one through Israel. The word "fullness" is found twice in Romans 11, once referring to Israel and once to the Church.
Romans 11:12 – If their loss… is riches for the Gentiles, how much more their fullness?
Concerning the Church:
Romans 11:25 – I do not want you to be ignorant of this mystery…, partial hardening has happened to Israel, until the fullness of the Gentiles has come in.
We believe in the dual restoration of Israel and the Church. They will both come into their fullness. In a mysterious way, the fullness of Israel is dependent on the Church, and the fullness of the Church is dependent on Israel. Although on different paths, they will both reach their fullness at the same time.
The path for the fullness of the Church ends up with a revelation about Israel (Romans 11:25). The path of fullness for Israel ends up with a revelation about Yeshua (Romans 11:26). When both come into their fullness, they are united into one. Then together they invite Yeshua to return (Matthew 23:39, Revelation 22:20), the dead are raised, and the kingdom of God is established on the earth.