From the Writings and Utterances of `Abdu’l-Baha:
The first teaching of Baha’u’llah is the duty incumbent upon all to investigate reality. What does it mean to investigate reality? It means that man must forget all hearsay and examine truth himself, for he does not know whether statements he hears are in accordance with reality or not. Wherever he finds truth or reality, he must hold to it, forsaking, discarding all else; for outside of reality there is naught but superstition and imagination.
(`Abdu’l-Baha: Promulgation of Universal Peace, p. 62)

The first principle of Baha’u’llah is independent investigation of truth, that is, all the nations of the world have to investigate after truth independently and turn their eyes from the moribund blind imitations of the past ages entirely. Truth is one when it is independently investigated, it does not accept division. Therefore the independent investigation of truth will lead to the oneness of the world of humanity.
(`Abdu’l-Baha: Japan Will Turn Ablaze, p. 35)

Man must cut himself free from all prejudice and from the result of his own imagination, so that he may be able to search for truth unhindered. Truth is one in all religions, and by means of it the unity of the world can be realized.
All the peoples have a fundamental belief in common. Being one, truth cannot be divided, and the differences that appear to exist among the nations only result from their attachment to prejudice. If only men would search out truth, they would find themselves united.
(`Abdu’l-Baha: Paris Talks, p. 129)

Likewise, the divine religions of the holy Manifestations of God are in reality one, though in name and nomenclature they differ. Man must be a lover of the light, no matter from what dayspring it may appear. He must be a lover of the rose, no matter in what soil it may be growing. He must be a seeker of the truth, no matter from what source it come. Attachment to the lantern is not loving the light. Attachment to the earth is not befitting, but enjoyment of the rose which develops from the soil is worthy. Devotion to the tree is profitless, but partaking of the fruit is beneficial. Luscious fruits, no matter upon what tree they grow or where they may be found, must be enjoyed. The word of truth, no matter which tongue utters it, must be sanctioned. Absolute verities, no matter in what book they be recorded, must be accepted. If we harbor prejudice, it will be the cause of deprivation and ignorance. The strife between religions, nations and races arises from misunderstanding. If we investigate the religions to discover the principles underlying their foundations, we will find they agree; for the fundamental reality of them is one and not multiple. By this means the religionists of the world will reach their point of unity and reconciliation. They will ascertain the truth that the purpose of religion is the acquisition of praiseworthy virtues, the betterment of morals, the spiritual development of mankind, the real life and divine bestowals.
(`Abdu’l-Baha: Promulgation of Universal Peace, pp. 151-152)

Furthermore, know ye that God has created in man the power of reason, whereby man is enabled to investigate reality. God has not intended man to imitate blindly his fathers and ancestors. He has endowed him with mind, or the faculty of reasoning, by the exercise of which he is to investigate and discover the truth, and that which he finds real and true he must accept. He must not be an imitator or blind follower of any soul. He must not rely implicitly upon the opinion of any man without investigation; nay, each soul must seek intelligently and independently, arriving at a real conclusion and bound only by that reality. The greatest cause of bereavement and disheartening in the world of humanity is ignorance based upon blind imitation. It is due to this that wars and battles prevail; from this cause hatred and animosity arise continually among mankind.
(`Abdu’l-Baha: Promulgation of Universal Peace, p. 291)

The purpose of this is to explain that the darkness of imitations encompasses the world. Every nation is holding to its traditional religious forms. The light of reality is obscured. Were these various nations to investigate reality, there is no doubt they would attain to it. As reality is one, all nations would then become as one nation. So long as they adhere to various imitations and are deprived of reality, strife and warfare will continue and rancor and sedition prevail. If they investigate reality, neither enmity nor rancor will remain, and they will attain to the utmost concord among themselves.
(`Abdu’l-Baha: Promulgation of Universal Peace, pp. 221-222)

The first principle of the Teaching of Baha’u’llah is:
The Search after Truth
If a man would succeed in his search after truth, he must, in the first place, shut his eyes to all the traditional superstitions of the past.
The Jews have traditional superstitions, the Buddhists and the Zoroastrians are not free from them, neither are the Christians! All religions have gradually become bound by tradition and dogma.
All consider themselves, respectively, the only guardians of the truth, and that every other religion is composed of errors. They themselves are right, all others are wrong! The Jews believe that they are the only possessors of the truth and condemn all other religions. The Christians affirm that their religion is the only true one, that all others are false. Likewise the Buddhists and Muhammadans; all limit themselves. If all condemn one another, where shall we search for truth? All contradicting one another, all cannot be true. If each believe his particular religion to be the only true one, he blinds his eyes to the truth in the others. If, for instance, a Jew is bound by the external practice of the religion of Israel, he does not permit himself to perceive that truth can exist in any other religion; it must be all contained in his own!
We should, therefore, detach ourselves from the external forms and practices of religion. We must realize that these forms and practices, however beautiful, are but garments clothing the warm heart and the living limbs of Divine truth. We must abandon the prejudices of tradition if we would succeed in finding the truth at the core of all religions. If a Zoroastrian believes that the Sun is God, how can he be united to other religions? While idolaters believe in their various idols, how can they understand the oneness of God?
It is, therefore, clear that in order to make any progress in the search after truth we must relinquish superstition. If all seekers would follow this principle they would obtain a clear vision of the truth.
If five people meet together to seek for truth, they must begin by cutting themselves free from all their own special conditions and renouncing all preconceived ideas. In order to find truth we must give up our prejudices, our own small trivial notions; an open receptive mind is essential. If our chalice is full of self, there is no room in it for the water of life. The fact that we imagine ourselves to be right and everybody else wrong is the greatest of all obstacles in the path towards unity, and unity is necessary if we would reach truth, for truth is one.
Therefore it is imperative that we should renounce our own particular prejudices and superstitions if we earnestly desire to seek the truth. Unless we make a distinction in our minds between dogma, superstition and prejudice on the one hand, and truth on the other, we cannot succeed. When we are in earnest in our search for anything we look for it everywhere. This principle we must carry out in our search for truth.
Science must be accepted. No one truth can contradict another truth. Light is good in whatsoever lamp it is burning! A rose is beautiful in whatsoever garden it may bloom! A star has the same radiance if it shines from the East or from the West. Be free from prejudice, so will you love the Sun of Truth from whatsoever point in the horizon it may arise! You will realize that if the Divine light of truth shone in Jesus Christ it also shone in Moses and in Buddha. The earnest seeker will arrive at this truth. This is what is meant by the `Search after Truth’.
It means, also, that we must be willing to clear away all that we have previously learned, all that would clog our steps on the way to truth; we must not shrink if necessary from beginning our education all over again. We must not allow our love for any one religion or any one personality to so blind our eyes that we become fettered by superstition! When we are freed from all these bonds, seeking with liberated minds, then shall we be able to arrive at our goal.
`Seek the truth, the truth shall make you free.’ So shall we see the truth in all religions, for truth is in all and truth is one!
(`Abdu’l-Baha: Paris Talks, pp. 135-137)

Likewise, when you meet those whose opinions differ from your own, do not turn away your face from them. All are seeking truth, and there are many roads leading thereto. Truth has many aspects, but it remains always and forever one.
Do not allow difference of opinion, or diversity of thought to separate you from your fellow-men, or to be the cause of dispute, hatred and strife in your hearts.
Rather, search diligently for the truth and make all men your friends.
(`Abdu’l-Baha: Paris Talks, p. 53)

The state in which one should be to seriously search for the truth is the condition of the thirsty, burning soul desiring the water of life, of the fish struggling to reach the sea, of the sufferer seeking for the true doctor to obtain the divine cure, of the lost caravan endeavoring to find the right road, of the lost and wandering ship striving to reach the shore of salvation.
Therefore, the seeker must be endowed with certain qualities. First of all, he must be just and severed from all else save God; his heart must be entirely turned to the supreme horizon; he must be free from the bondage of self and passion, for all these are obstacles. Furthermore, he must be able to endure all hardships. He must be absolutely pure and sanctified, and free from the love or the hatred of the inhabitants of the world. Why? because the fact of his love for any person or thing might prevent him from recognizing the truth in another, and, in the same way, hatred for anything might be a hindrance in discerning truth. This is the condition of seeking, and the seeker must have these qualities and attributes.
(`Abdu’l-Baha: Some Answered Questions, pp. 38-39)

In accordance with the divine teachings in this glorious dispensation we should not belittle anyone and call him ignorant, saying: `You know not, but I know’. Rather, we should look upon others with respect, and when attempting to explain and demonstrate, we should speak as if we are investigating the truth, saying: `Here these things are before us. Let us investigate to determine where and in what form the truth can be found.’
(`Abdu’l-Baha: Selections … `Abdu’l-Baha, p. 30)

From Letters Written on Behalf of Shoghi Effendi:
The basic principle of the Cause is independent investigation of truth. This applies to us as much as to our children. They should be free to choose for themselves any religion they wish.
(Shoghi Effendi: Lights of Guidance, p. 156)