The prophet and founder of the Baha’i Faith, Baha’u’llah, said clearly that God has two main reasons for establishing religion:

God’s purpose in sending His Prophets unto men is twofold. The first is to liberate the children of men from the darkness of ignorance, and guide them to the light of true understanding. The second is to ensure the peace and tranquility of mankind, and provide all the means by which they can be established.

Baha’u’llah

One of the ways to understand the oneness of all the Prophets of God is to consider their most compelling and profound inner messages. If you look at those central messages with wisdom and insight, you can see the utterances and teachings of these Manifestations of God all express a single truth.
Abdu’l-Bahá explains:

We are considering the divine plan for the reconciliation of the religious systems of the world. Bahá’u’lláh has said that if one intelligent member be selected from each of the varying religious systems, and these representatives come together seeking to investigate the reality of religion, they would establish an interreligious body before which all disputes and differences could be presented for consideration and settlement. Such questions could then be weighed and viewed from the standpoint of reality and all imitations discarded. By this method and procedure all sects, denominations and systems would become one.

Abdu’l-Bahá