Jesus taught that we are to love one another but Jesus did not teach the unity of mankind. Unity of mankind would not have been possible 2000 years ago when Jesus walked the earth because the whole world had not even been discovered and people all over the world were certainly not connected, as we are now.
Religion and Social Evolution
The human race needs more than the message that we are to love one another, that is old news. Baha'u'llah came to bring justice.
"The best beloved of all things in My sight is Justice; turn not away therefrom if thou desirest Me, and neglect it not that I may confide in thee." The Hidden Words of Bahá’u’lláh, pp. 3-4
Justice means punishing actions or words that are wrong and upholding things that are good. This helps ensure that wrongs will be ended and rights will be upheld thereby leading to a safer society for everyone. It would seem sometimes that children have a keen internal sense of justice.
What is justice and why is it important? - Quora
We can see why justice is so important if we look at what is going on in the world today. Is love going to solve any of these problems? Sure, we need to love one another, but we need more than that. We need justice.
Jesus prayed that someday we may be one but Jesus did not teach the unity of mankind. That is a new message, the message of Baha'u'llah.
“The Revelation associated with the Faith of Jesus Christ focused attention primarily on the redemption of the individual and the molding of his conduct, and stressed, as its central theme, the necessity of inculcating a high standard of morality and discipline into man, as the fundamental unit in human society. Nowhere in the Gospels do we find any reference to the unity of nations or the unification of mankind as a whole. When Jesus spoke to those around Him, He addressed them primarily as individuals rather than as component parts of one universal, indivisible entity. The whole surface of the earth was as yet unexplored, and the organization of all its peoples and nations as one unit could, consequently, not be envisaged, how much less proclaimed or established.”
The Promised Day Is Come, p. 119