Before entering into the merits of our question, we bring a reflection by Herculano Pires, who claims to speak all of Spiritism, good or bad, but few know it, generally considering it a common religious sect, full of superstitions. He points out that many see him as a way of systematizing popular beliefs, and there are also those who accept him as a new Goce, the black magic of antiquity disguised as miracle Christianity, that is, Spiritism, born in the middle of the last century, is still today the Great Unknown of those who approve and praise him, as well as those who attack and criticize him.
We observe that in the time of Kardec, as still today, those who know nothing of Spiritism tend to take it for what it is not, a religion or sect with its mysteries, supposed priests dedicated to consult in the gloom the dead for the purposes more varied. This is exactly why Kardec invites us to study the Spiritist Doctrine, emphasizing that Spiritists need to know Spiritism well, to study it well, to build in the soul the philosophical principles that claim to adopt and continually seek spiritual progress.
In this question we will address the threefold aspect of the Spiritist Doctrine, a matter of great importance for those who really want to understand about Spiritism, and as such, this is only possible through serious and methodical study.
Spiritism is science, philosophy and religion, which is its triple aspect, since research techniques were used to prove the truth of the spiritist phenomena (science) and, based on the revelation of results (truths), questions were formulated with a high philosophical content (philosophy), verifying that the application of those can be used in the moral transformation of Man (religion).
Science, by the dictionary, is deep knowledge about something. Knowledge or excessive knowledge gained by practice, reasoning or reflection. Meeting of organized knowledge obtained by observation, research or the demonstration of certain events, facts, phenomena, being systematized by methods or in a rational way: the norms of science.
Spiritism is science because it studies, in the light of reason and within scientific criteria, the mediumistic phenomena, that is to say, phenomena provoked by the spirits and that they are nothing more than natural facts and absolutely within the divine laws. All phenomena, even the strangest, have a scientific explanation. There is no supernatural in Spiritism. Spiritism, being a science of observation, studies and analyzes the relations of the Spirits with the corporeal world, as Kardec once stated in the book "What is Spiritism", being well explained and described in The Book of Mediums his research methodology, noting has been a sure way of dealing with spirits in serious meetings.
Through this methodology, which we will know more in detail in the question "How did Allan Kardec codify the Spiritist Doctrine?", The veil that concealed part of the perception of the invisible world was taken away and made us aware of the immortality of the soul, evidenced by the innumerable facts that mediumship demonstrates. Kardec evoked Higher Spirits to question them on philosophical subjects of great depth, through mediums acting in the Society, at no time considering any Spirit as exclusive and privileged revealer, nor the theses revealed as absolute truths.
He compared the teachings, both those gathered at meetings in Paris and those from correspondence with other groups, received through mediums unknown to the first ones, the Spiritist being presented as progressive on the Kardecian basis and dialogical in relation to the ordinary sciences. In this sense the noted teacher pointed out that Spiritism would always march with progress, reason why it would never be exceeded, because if new discoveries prove to be in error on a certain point, it would change on that point, that is to say, if a new truth to reveal itself, he would accept it. 
The scientific aspect of Spiritism is developed in two works by Allan Kardec: The Mediums Book and The Genesis.
Spiritism is philosophy because it analyzes Divine Creation, explaining the reason God created man, his origin and destination, reflects on the causes of human happiness and unhappiness. Questions such as: "Where I came from," "what I do in the world," "where I will go after death" are fully answered by Spiritism, noting that every doctrine that gives an interpretation of life, a conception of the world, is a philosophy.
The Spiritist Doctrine is a philosophy that teaches self-knowledge as an indispensable tool for the intellectual-moral evolution of the Spirit, having a profound potential to transform the world from the individual. "Its force is in its philosophy, in the appeal that it directs to reason, to the common sense".
Therefore, the philosophical aspect portrays the principles and teachings of the Superior Spirits collected and organized by Allan Kardec, plus the contribution of the noble encoder obtained in his reflections and studies, being this philosophical aspect approached in The Book of the Spirits.
Spiritism is religion because it triggers, from the appreciation of the principles it brings, a moral that leads man to the ethics of love exemplified by the Divine Master. Let us remember that morality can be seen as synonymous with ethics, as a theory of values that regulate human conduct.
The Spiritist Doctrine aims at the moral transformation of man, reviving the teachings of Jesus Christ in his true expression of simplicity, purity and love, seeking a simple religion, without priests, ceremonies and no sacraments of any kind, without rituals, worship images, candles, special garments and no exterior manifestations.
The religious aspect will also address the moral consequences of human behavior, defined by the use of free will and governed by the law of cause and effect, to which all mankind is subject. Spiritist morality makes man a good person, knowing how to distinguish good from evil and choose the first, since the true good man, as the Gospel According to Spiritism teaches, "is what fulfills the law of justice, of love and of charity, in its highest purity. " 
Taking this orientation as reference, the Spirit Emmanuel elucidated we can take the Spiritism symbolized as a triangle of spiritual forces, linking Science and Philosophy to Earth and Religion to heaven as the divine angle. It also adds that the Doctrine, in its scientific and philosophical aspect, will always be a noble field for human investigations, aiming at the perfection of Humanity, but in the religious aspect, however, rests its divine greatness, since it constitutes the restoration of the Gospel of Jesus Christ, promoting the definitive spiritual renewal of man.  The religious aspect of the Spiritist Doctrine is developed by Allan Kardec in the basic works The Gospel According to Spiritism and Heaven and Hell.
Id. O Evangelho Segundo do Espiritismo. 131 ed. Brasília: FEB, 2013. p. 232.
XAVIER, Francisco Cândido. O consolador. Pelo espírito Emmanuel. [s.l.]:FEB, [2010?]. p. 9.
KARDEC, Allan. A Gênese. 53 ed. Brasília: FEB, 2013. p. 41.
KARDEC, Allan. O Livro dos Espíritos. 93 ed. Brasília: FEB, 2013. p. 464.
PIRES, Herculano. Curso Dinâmico de Espiritismo – O grande desconhecido. São Paulo: Paideia, [2000?]p. 11.
Portal Dicio. Dicionário Online de Português. Disponível em:<https://bit.ly/2MyArqM>. Acesso em: 7 jun. 2018.