Before the discussion begins on the forbidden tree which was situated in the garden where Prophet Adam (as) was sent to, it should be emphasized that this is not an essay on the fallibility of Prophet Adam (as). Adam’s act of disregarding the advice by Allah (swt) is commonly termed as ‘selecting a less preferred option’ (tark al-awla). The belief that the Prophets (s) and Imams (as) are infallible is fundamental to the followers of the AhlulBayt (as).
After the intrusion from Iblis (satan), the act of Prophet Adam (as) with respect to the tree and to eat from it has been marked as an act of inclining to multiplicity in the book Adabus Salat by Imam Ruhullah Khomeini. Paying attention to the tree, which is the tree of multiplicity, and eating from it has many degrees and manifestations. Since Prophet Adam (as) was taught all the names by Allah (swt) (holy Quran, 2:31), it was expected that he would manifest the unity of names in all his actions, but when he turned his attention towards the forbidden tree, he actually turned from the unity towards the multiplicity. The unity of names, attributes and actions refers to every knowledge, intention and action which is performed for the sake of Allah (swt). On the other hand, multiplicity of names refers to paying attention to the individual fragments and thereby forgetting or neglecting Allah (swt), according to Imam Khomeini.
Irrespective of the different affairs in the life of a man, if the believer is content with what Allah (swt) has decreed, then the believer is manifesting the unity of names because he is aware of the sacred presence of his Lord. But as soon as he turns his attention to the love of the world and the love of the self then he moves from the unity to the multiplicity. And the multiplicity is the innate nature of the forbidden tree.
It may be said that all kinds of disobedience of the people could be related to the eating from today’s tree which is the multiplicity of the present world. All the sins of the soul are also related to the same tree. Always, we are occupied with selfish motives and desires of the world which have individualities and are taking us away from Allah. These individualities are nothing but the fruits of the tree. The fruit appears to be attractive and delicious much like the whims and desires of the world but after taking a bite from it, the taste of wickedness takes us away from Allah (swt). We are not as wise as Prophet Adam (as) that we are mindful of the multiplicity of today’s world and ask for repentance. Therefore, we should sincerely repent and ask Allah (swt) to turn us towards His unity from all the multiplicity of this world. This will be a manifestation of monotheism (tawhid).
It is observed that even the so called ‘strictly practicing Muslims’ are affected by the disease of multiplicity especially in moments of trivial obstacles. Worrying over the battery of the mobile phone, queuing up for the latest gadget or a fiction book, wearing expensive jerseys of the bogus sports clubs, looking up to the lives of celebrities, succumbing to the pressure of shaking hands with a non-mahram, and competing with fellow Muslims are only very few of the examples that take people towards multiplicity. These are nothing but the fruits of the tree which Iblis tries to turn our attention to and let us fall from Divine unity. Firstly, we should try not to be a prisoner of the multiplicity and secondly, try to focus on uniting our halal actions and submitting them to Allah (swt) only.
With respect to the tree from where Adam had eaten, there is a noble tradition to the effect that the Jews asked the Holy Prophet: “What caused Allah to impose on your people to fast for thirty days?” He said: “It was Adam (as), because what he had eaten from that tree remained in his stomach for thirty days. So Allah made it incumbent upon him and his offspring to endure hunger and thirst for thirty days, and He allowed them, out of His kindness, to eat and drink at the nights.” 
We should avoid our animalistic desires and purge our soul by purifying it from Satanic whims. One way to do this is to keep in mind that paying attention to anything other than Allah (swt) is the root of the forbidden tree. This does not mean we cannot keep up with worldly affairs–but that worldly affairs are not the ends, only the means to the ends which is Allah (swt). Imam Khomeini (may Allah be pleased with him) writes that we must know that eating of this tree of nature, desiring this world and attending to multiplicity are the origins of major impurity (janaabah).
Imam Ali (as) has said: “The perfect monotheism is the negation of ascribing attribute to Him.”  The negation of ascribing attributes denotes otherness (ghayriyyat) and plurality to Allah (swt). All the attributes of Allah (swt) are in inclusive unity and they cannot be individually fragmented in a sense that one attribute becomes absent and the other is present.
Therefore, the act of Adam could be an inclination towards the plurality of names, which was the spirit of the forbidden tree. Even if we are performing all the religious duties, we should not be satisfied with our limited knowledge and try to purify our hearts and focus our lifestyles to submit to the state of unity. This could be done by recognizing our need of the absolute powerful and remaining humble in His (swt) presence with repentance and dhikr. Seeking the help of the Seal of the Prophets (s) and his pure Household (as), our sins would be easily washed away if we are ready to clear the dust of the mundane multiplicity from our eyes. Otherwise the reality of unity would never make an impression on our hearts and we will always be lured by the fruits of the forbidden tree. Ilal ash-Sharai, Volume. 2, 109, 1.
 Usul al-Kafi, Volume 1, pg 191.