"One Common Faith" means Baha'is will eradicate all religions and convert everyone to Baha'ism:
"We desire but the good of the world and the happiness of the nations; that
all nations shall become one in faith and all men as brothers; that the bonds of affection and unity between the sons of men shall be strengthened; that
diversity of religion shall cease and differences of race be annulled. So it shall ,be; these fruitless strifes, these ruinous wars shall pass away, and the 'Most Great Peace' shall come (Baha'u'llah)." (Star of the West, Vol. 3, no. 7)
http://starofthewest.info/Vol03/large/SOTW_Vol03_129.gif
However what Baha'is will never tell you is that this conversion is not meant to come by peaceful means but rather in a genocidal form. Three translations are provided below of what Baha'u'llah's writes in
Āthār-i Qalam-i A`lā, vol. 2, no. 90, p. 587
"God will soon take out from the sleeves of power the hands of strength and dominance and will make the Servant (Bahā’u’llāh) victorious and will cleanse the earth from the filth of every rejected polytheist (deniers of Baha’ism). And they will stand by the cause and will conquer the lands using my mighty eternal name and will enter the lands and they will be feared by all the servants." (Translation by Hossein Akhoondali, Twelve Principles book)
"Erelong shall God draw forth, out of the bosom of power, the hands of ascendancy and might, and shall raise up a people who will arise to win victory for this Youth and who will purge mankind from the defilement of the outcast and the ungodly. These hands will gird up their loins to champion the Faith of God, and will, in My name the Self-Subsistent, the Mighty, subdue the peoples and kindreds of the earth. They will enter the cities and will inspire with fear the hearts of all their inhabitants." (Translation from The Summons of the Lord of Hosts)
"God shall send forth from the sleeves of Might the hands [Hands] of power and victory, and certain people to assist this Servant, and to cleanse the earth from the impurity of every abandoned polytheist, and they shall arise in favor of this matter, and open the countries in My Name, the Powerful, the Self-existent, and enter their boundaries, and their threats shall overtake all the servants" (Translated by Anton Haddad)
There is a reason that Baha'is are ordered to do this:
"Be thou as a flame of fire to My enemies and a river of life eternal to My loved" (Baha'u'llah)
and this:
“And you, O friends of God, be clouds of grace for those who believe in God and his signs, and be certain torment for those who do not believe in God and are polytheists (deniers of Baha’ism),” Baha’u’llah, Majmu`iy-i alwah-i mubarak-ih, p. 216.