Often in the past religion has caused strife. Sometimes co-religionists have fought about religious disagreement; sometimes men have attacked those of a different faith to force acceptance of their own belief. Still more often, differences of religion or sect have caused hatred, division and dislike, without physical violence. Even to-day there are millions of people who try to make others unhappy because they do not approve of their religious beliefs. Many more contribute to discord by narrow insistence on their religious view and refusal to investigate the views of others.
A casual glance at the great religions of the world is enough, if unprejudiced, to reveal that they have much in common. All teach that we should love one another, do good, be sincere, truthful and law-abiding; that we should seek out our own shortcomings before we presume to condemn the faults of others, and that we should not consider ourselves superior to our neighbours. Each has a Central Figure, who is revered above all men and a scripture based on His teaching. With so much in common, it is strange that the followers of different religions should be so antagonistic to each other’s beliefs.
The point has been well made by George Townshend in his book The Promise of All Ages:
“All the world over, mankind has honoured the spokesmen of God and has adopted their teachings. It reveres Christ, Buddha, Zoroaster, Krishna, and other High-Prophets as its greatest leaders. But it has not looked on them as related to one another. It has thought of them as rivals, competing for the homage of the world. It has imagined that to accept the revelation of one is to deny the revelation of every other and that the votaries of any one High-Prophet are not loyal to their Lord unless they esteem him the sole authentic revealer come from God. It has balanced the High-Prophets against one another as it were in scales, so that when one goes up, the others must go down; . . . Thus the influence of religion, which ought to have tended to unify the peoples of the world, has through a misunderstanding engendered hostility and strife. The High-Prophets never spoke ill of one another: the antagonism originated with their followers.. . . None affirmed that his revelation was final or exhaustive; . . .” 
The followers of the Great Leaders have caused differences between religions to appear which had no part in the Message of their Founders. The Founders preached unity and concord because, although each revealed His Message in a form suited to the particular needs of a particular age, their Revelation was in essence one. Bahá’u’lláh, the Central Figure of the Bahá’í Faith, explains this in a beautiful passage:
“Their revelation may be likened unto the light of the moon that sheddeth it radiance upon the earth. Though every time it appeareth, it revealeth a fresh measure of its brightness, yet its inherent splendour can never diminish, nor can its light suffer extinction.
It is clear and evident, therefore, that any apparent variation in the intensity of their light is not inherent in the light itself, but should rather be attributed to the varying receptivity of an ever-changing world. Every Prophet Whom the Almighty and Peerless Creator hath purposed to send to the peoples of the earth hath been entrusted with a Message, and charged to act in a manner that would best meet the requirements of the age in which He appeared.” 
Needs change, and the needs of yesterday are not the needs of today; laws that were fitting when mankind was young would not be fitting now. Progress decrees that the form of religion revealed for an earlier stage of man’s development cannot suit the later stages too. Under the tuition of each of the Great Educators humanity advances, so that the next Manifestation of God comes to men prepared to hear what before was hidden.
It is as though man ascended through the classes of a school; each successive teacher imparts more truths to the child and when he has digested these he is ready to move to the next class. The lesson is suited to the child’s stage of progress; there would be no advantage in giving a sixth-form lesson to a first-form pupil, rather the too great demand on the undeveloped soul would cause it to wither and die. So also the teaching of the Great educators is mercifully adapted to the stage of development reached by mankind.
The Messenger of God reveals both spiritual truths, which are eternal, and laws belonging to a particular age. The spiritual truths are revealed according to the spiritual development of the men of that time. Thus Moses said: “Love thy neighbour as thyself,"  but only the rarer spirits in His dispensation realised that Gentiles also were their neighbours. Jesus stressed that love should extend beyond the Jewish race, but still His followers were unable to grasp fully the oneness of mankind. Only recently have men progressed enough to regard the whole human race as one family, without division of colour, class or creed. Bahá’u’lláh, coming to a world prepared by the long line of earlier Messengers of God, could make this a central feature of His Teaching. All three Messengers were aware of the truth taught by Bahá’u’lláh, but until now man has not been ready to receive its full force. Knowing the limitations of the men of His age, Jesus said:
Besides such eternal teachings gradually unfolded to the advancing spirit of man, each Messenger of God also brings temporary laws and commands. These may be changed by the next of God’s Messengers. To the Tribes of Israel wandering in the desert, Moses gave many laws which Jesus in a later age repealed. Yet even as He repealed them Jesus proclaimed: “Think not that I am come to destroy the law, or the prophets; I am not come to destroy, but to fulfil.”  The truth that shines through the Law of Moses is revealed also in the Law of Jesus; but circumstances in the world had changed and a new law was needed to fulfil the old.
When the Israelites were in the desert, they had no prisons and no established home. Had Moses not ordained strict laws with fearful punishments, the innocent among Israel would have been at the mercy of the vicious. Love and Justice demanded that the law be strong. Therefore Moses decreed death as the penalty for ten separate offences. “An eye for an eye, and a tooth for a tooth ”  was in those days a merciful command, prohibiting excessive punishment. Later, when Jesus came, altered circumstances permitted the law of love to be expressed in gentler form. ‘Abdu’l-Bahá, Son of Bahá’u’lláh, has explained this in the following words:
“The practical part of religion deals with exterior forms and ceremonies, and with modes of punishment for certain offences. This is the material side of the law, and guides the customs and manners of the people.
In the time of Moses, there were ten crimes punishable by death. When Christ came this was changed; the old axiom, ‘an eye for an eye and a tooth for a tooth,’ was converted into ‘Love your enemies, do good to them that hate you’, the stern old law being changed into one of love, mercy and forbearance!
In the former days the punishment for theft was the cutting off of the right hand; in our time this law could not be so applied. In this age, a man who curses his father is allowed to live, when formerly he would have been put to death. It is therefore evident that whilst the spiritual law never alters, the practical rules must change their application with the necessities of the time. ” 
To change these rules, and to give man more advanced teaching, God sends His Messengers to walk upon the earth.