From the Utterances of `Abdu’l-Baha:
Baha’u’llah declared that religion is in complete harmony with science and reason. If religious belief and doctrine is at variance with reason, it proceeds from the limited mind of man and not from God; therefore, it is unworthy of belief and not deserving of attention; the heart finds no rest in it, and real faith is impossible. How can man believe that which he knows to be opposed to reason? Is this possible? Can the heart accept that which reason denies? Reason is the first faculty of man, and the religion of God is in harmony with it.
(`Abdu’l-Baha: Promulgation of Universal Peace, p. 231)

The third teaching or principle of Baha’u’llah is that religion and science are in complete agreement. Every religion which is not in accordance with established science is superstition. Religion must be reasonable. If it does not square with reason, it is superstition and without foundation. It is like a mirage, which deceives man by leading him to think it is a body of water. God has endowed man with reason that he may perceive what is true.
(`Abdu’l-Baha: Promulgation of Universal Peace, p. 63)

The third principle or teaching of Baha’u’llah is the oneness of religion and science. Any religious belief which is not conformable with scientific proof and investigation is superstition, for true science is reason and reality, and religion is essentially reality and pure reason; therefore, the two must correspond. Religious teaching which is at variance with science and reason is human invention and imagination unworthy of acceptance, for the antithesis and opposite of knowledge is superstition born of the ignorance of man. If we say religion is opposed to science, we lack knowledge of either true science or true religion, for both are founded upon the premises and conclusions of reason, and both must bear its test.
(`Abdu’l-Baha: Promulgation of Universal Peace, p. 107)

Third, that religion is in harmony with science. The fundamental principles of the Prophets are scientific, but the forms and imitations which have appeared are opposed to science. If religion does not agree with science, it is superstition and ignorance; for God has endowed man with reason in order that he may perceive reality. The foundations of religion are reasonable. God has created us with intelligence to perceive them. If they are opposed to science and reason, how could they be believed and followed?
(`Abdu’l-Baha: Promulgation of Universal Peace, p. 128)

Furthermore, the teachings of Baha’u’llah announce that religion must be in conformity with science and reason; otherwise, it is superstition; for science and reason are realities, and religion itself is the Divine Reality unto which true science and reason must conform. God has bestowed the gift of mind upon man in order that he may weigh every fact or truth presented to him and adjudge whether it be reasonable. That which conforms to his reason he may accept as true, while that which reason and science cannot sanction may be discarded as imagination and superstition, as a phantom and not reality. Inasmuch as the blind imitations or dogmatic interpretations current among men do not coincide with the postulates of reason, and the mind and scientific investigation cannot acquiesce thereto, many souls in the human world today shun and deny religion. That is to say, imitations, when weighed in the scales of reason, will not conform to its standard and requirement. Therefore, these souls deny religion and become irreligious, whereas if the reality of the divine religions becomes manifest to them and the foundation of the heavenly teachings is revealed coinciding with facts and evident truths, reconciling with scientific knowledge and reasonable proof, all may acknowledge them, and irreligion will cease to exist. In this way all mankind may be brought to the foundation of religion, for reality is true reason and science, while all that is not conformable thereto is mere superstition.
(`Abdu’l-Baha: Promulgation of Universal Peace, pp. 373-374)

Religion and science are the two wings upon which man’s intelligence can soar into the heights, with which the human soul can progress. It is not possible to fly with one wing alone! Should a man try to fly with the wing of religion alone he would quickly fall into the quagmire of superstition, whilst on the other hand, with the wing of science alone he would also make no progress, but fall into the despairing slough of materialism. All religions of the present day have fallen into superstitious practices, out of harmony alike with the true principles of the teaching they represent and with the scientific discoveries of the time. Many religious leaders have grown to think that the importance of religion lies mainly in the adherence to a collection of certain dogmas and the practice of rites and ceremonies! Those whose souls they profess to cure are taught to believe likewise, and these cling tenaciously to the outward forms, confusing them with the inward truth.
Now, these forms and rituals differ in the various churches and amongst the different sects, and even contradict one another; giving rise to discord, hatred, and disunion. The outcome of all this dissension is the belief of many cultured men that religion and science are contradictory terms, that religion needs no powers of reflection, and should in no wise be regulated by science, but must of necessity be opposed, the one to the other. The unfortunate effect of this is that science has drifted apart from religion, and religion has become a mere blind and more or less apathetic following of the precepts of certain religious teachers, who insist on their own favourite dogmas being accepted even when they are contrary to science. This is foolishness, for it is quite evident that science is the light, and, being so, religion truly so-called does not oppose knowledge.
We are familiar with the phrases `Light and Darkness’, `Religion and Science’. But the religion which does not walk hand in hand with science is itself in the darkness of superstition and ignorance.
Much of the discord and disunion of the world is created by these man-made oppositions and contradictions. If religion were in harmony with science and they walked together, much of the hatred and bitterness now bringing misery to the human race would be at an end.
Consider what it is that singles man out from among created beings, and makes of him a creature apart. Is it not his reasoning power, his intelligence? Shall he not make use of these in his study of religion? I say unto you: weigh carefully in the balance of reason and science everything that is presented to you as religion. If it passes this test, then accept it, for it is truth! If, however, it does not so conform, then reject it, for it is ignorance!
Look around and see how the world of today is drowned in superstition and outward forms!
Some worship the product of their own imagination: they make for themselves an imaginary God and adore this, when the creation of their finite minds cannot be the Infinite Mighty Maker of all things visible and invisible! Others worship the sun or trees, also stones! In past ages there were those who adored the sea, the clouds, and even clay!
Today, men have grown into such adoring attachment to outward forms and ceremonies that they dispute over this point of ritual or that particular practice, until one hears on all sides of wearisome arguments and unrest. There are individuals who have weak intellects and their powers of reasoning have not developed, but the strength and power of religion must not be doubted because of the incapacity of these persons to understand.
A small child cannot comprehend the laws that govern nature, but this is on account of the immature intellect of that child; when he is grown older and has been educated he too will understand the everlasting truths. A child does not grasp the fact that the earth revolves round the sun, but, when his intelligence is awakened, the fact is clear and plain to him.
It is impossible for religion to be contrary to science, even though some intellects are too weak or too immature to understand truth.
God made religion and science to be the measure, as it were, of our understanding. Take heed that you neglect not such a wonderful power. Weigh all things in this balance.
To him who has the power of comprehension religion is like an open book, but how can it be possible for a man devoid of reason and intellectuality to understand the Divine Realities of God?
Put all your beliefs into harmony with science; there can be no opposition, for truth is one. When religion, shorn of its superstitions, traditions, and unintelligent dogmas, shows its conformity with science, then will there be a great unifying, cleansing force in the world which will sweep before it all wars, disagreements, discords and struggles – and then will mankind be united in the power of the Love of God.
(`Abdu’l-Baha: Paris Talks, pp. 143-146)

Furthermore, He [Baha’u’llah] proclaims that religion must be in harmony with science and reason. If it does not conform to science and reconcile with reason, it is superstition. Down to the present day it has been customary for man to accept a religious teaching, even though it was not in accord with human reason and judgment. The harmony of religious belief with reason is a new vista which Baha’u’llah has opened for the soul of man.
(`Abdu’l-Baha: Promulgation of Universal Peace, p. 455)