We are living in the age of the Gospel – that is, the period of time following the spiritual salvation provided through Jesus Christ. In the advent of Jesus, the Kingdom of Heaven made its arrival (Matthew 4:17, 11:2-6). However, the Kingdom of Heaven (obviously) has not yet been physically established here on earth; as a result, God's Kingdom exists on Earth in a manner that most theologians refer to as "Now & Not Yet". Until the second coming of Jesus, we live in a period that is actually the overlapping of two periods; we simultaneously live in the in the age of Fallen Humanity & Sin and the age of the Kingdom of Heaven.
This truth is made evident by the presence of spiritual warfare; the Church’s ongoing battle against the old order of worldliness reveals the presence of two competing forces (Ephesian 6:10-20). When the new age came to exist on earth through Jesus, the old age did not dissipate automatically; in fact, it still exists despite the fact that the regeneration of all things is already happening. This is why we struggle with the ways of the world (Romans 12:2, James 1:27), the flesh (Gal 5:17), and the Enemy (1 Peter 5:8-9); however, as part of the new creation in Jesus Christ, the Church is called to put to death whatever belongs to our sinful earthly nature (Colossians 3:3-5).
I once heard this overlapping age of war likened to World War II. The German forces were truly defeated on D-Day (when the Allies stormed French beaches) and the following seizure of Berlin; however, the fighting did not stop – but continued for 11 months until Germany surrendered. During the period in between, Hitler still waged war and launched attacks on the Allies, but was losing power as the Allies were gaining it. The Allied forces struggled for dominance while Nazi Germany struggled to maintain existence. The same is true of the old order of things and the new order in Jesus; the victory was established at the cross, but war will rage on until the complete dominance of the Kingdom of God that will be establish in the Second Coming of Christ.
That said, the Second Coming of Christ is a necessary consummation of His redeeming work. The resurrection does fulfill God’s covenant promise (Hebrews 13:20, Acts 2:29-35, 13:32-35) and reveal Jesus Christ as Lord (Acts 2:36), but it does not usher in the promised physical presence of God’s Kingdom on the recreated Earth (Isaiah 65:17, 2 Peter 3:13). This makes the Second Coming necessary as a consummation of the redemptive revelation, as the return of Jesus will end “the overlap of the ages and make the reality of the kingdom universally visible.” (Graeme Goldsworthy)
We live in a simultaneous age – that of the Old Order dissipating, and the New Order gaining dominance. The age of the gospel overlaps with the age to come, creating an ongoing spiritual battle between the Kingdom of Heaven and the Kingdom of the World. Thus, in order to consummate the redemption of all creation and establish the physical Kingdom of Heaven on New Earth, the return of Jesus Christ is necessary!