To begin, Baha'u'llah is pointing out a simple, "unquestionable" truth: the teachings, laws, commandments and prohibitions that were previously taught have ceased to exert their influence. The rest of this paragraph gives a singular example of this.
Many questions arise from this first sentence, though. First is why are those four listed in that order? It's a great question, but we will not spend time here looking at it, as we have done similar examinations previously.
Second, He talks about these aspects ceasing to exert their influence. Now, it is obvious that these various teachings and aspects of religion still exert influence, but He says that they no longer exert "their" influence. Is He implying that they exert something else's influence? If we look at the idea that these teachings are like the sun, then they should give light and warmth, and promote healthy growth. In a spiritual sense, these teachings should help us grow in compassion and love, lead us to greater knowledge of the world around us, and help promote higher degrees of unity. If there is anything in them that leads us towards hatred, or disunity, or closing off to the knowledge of the world, then we can be sure that we have misunderstood.
One spiritual teacher said that a void of the spiritual can lead to fanaticism. When thinking about this, we realize that if we see ourselves becoming fanatical then we can be certain that we have left spirituality behind.
As soon as we begin to think that we are right and have some sort of superior understanding to others, we have left that spirituality behind, for we have ceased to be humble. When we claim that our religion is the only one, or that another faith is somehow invalid, we have left that spirituality behind, for we have again ceased to be humble. When we deny another the freedom of choice, that God-given right to independent investigation, claiming that they will discover that we are correct at some indeterminate point in the future, we have truly left that spirituality behind along with our humility. But, when we listen to others, with an ear to searching for the truths that they have learned, which we may have missed, then we reclaim that spirit. "Far from challenging the validity of any of the great revealed faiths," writes the Universal House of Justice in their preface to One Common Faith, when referring to the interfaith tradition, "the principle has the capacity to ensure their continuing relevance." They themselves freely promote the truth that all religious teachings still have continuing relevance.
One example to look at, just in case we need it, is that of prayer. When we recite the Lord's Prayer, or even the Short Obligatory Prayer, those Words can raise us up to the heights of spiritual contemplation. Or, if we merely recite them out of obligation, we can feel proud for having fulfilled our duty. In that latter case they have ceased to exert their influence, and are, instead, promoting our own selfish desire.
The next point we want to look at is the interesting phrase, "consider now". This is in contrast to the earlier phrase, "consider the past". Up until this point, Baha'u'llah has not said anything against the Muslims of His day, for the man to whom He is writing would be in that category, and He obviously does not wish to offend. Here, Baha'u'llah has left off the admonition to consider the past, and is now asking this man to consider the present. But what does He do? He brings up an example of the past. He mentions the Christians denying Muhammad, and by allusion, slowly and carefully draws this man closer and closer to the present day problems with the Muslims striving to accept the Bab.
In short, there are many people who believe in the immutability of their religion, and base their understanding on the Words of the Founder of their faith. For example, Jesus, in Mark 13:31, said, "Heaven and earth will pass away, but My words will not pass away." And these people presume that it means the literal words on the paper, as opposed to the divine spirit contained within those words on paper. They fight, tooth and nail, to defend the least bit of their understanding, failing to see that the very stance of aggressiveness is contrary to the spirit that Jesus taught. They fail to recognize that the word of which Jesus was likely speaking is that same Word that was mentioned in the beginning of John, "In the beginning was the Word..."
When the Buddha said, "Everything changes, nothing remains without change", He gave us an insight into this truth which Baha'u'llah is alluding to here.
Baha'u'llah, in this paragraph, refers to those who have perished. They have died because they got stuck in the valley of waywardness and misbelief. And we can easily see ourselves stuck in this same position. But, with infinite grace, Baha'u'llah ends this paragraph with a glimmer of hope for us. He reminds us that we can get out of it by searching for that promised Sun that is rising, that same Sun that may be hidden by the hills surrounding us.