Ps 2: 6-8 "Yet have I set my king upon my holy hill of Zion. I will declare the decree: the LORD hath said unto me, Thou art my Son; this day have I begotten thee. Ask of me, and I shall give thee the heathen for thine inheritance, and the uttermost parts of the earth for thy possession. Thou shalt break them with a rod of iron; thou shalt dash them in pieces like a potter's vessel."
Rev. 19:15 "And out of his mouth goeth a sharp sword, that with it he should smite the nations: and he shall rule them with a rod of iron: and he treadeth the winepress of the fierceness and wrath of Almighty God."
Do you believe there is a 1000 year reign of Christ prior to the new heavens and new earth (Rev. 20)? Will the world then be full of people who are not born again believers (but some of them become such)? It says at the end of that 1000 years, Satan will be let loose and will deceive the nations, which indicates that mankind as a whole will still be under sin. Then with corruption of the sinful nature still in the world in those years, it will require Jesus ruling strongly to carry out what God wants to accomplish, bringing "many sons to glory" (possibly a million times more than currently exist). The scripture says He will rule with a rod of iron, and I believe it.