Gemara: [If Shlomo did not sin, then] what does it mean that "It was when Shlomo grew old, his wives turned his heart after other gods" (Melachim I, 11:3)? It is as Rav Natan said, as he brought a contradiction between the above and "His heart was not like that of his father David" (ibid.), which implies that he did not actually sin. The explanation is that they turned his heart toward following other gods, but he did not follow.
Ein Ayah: It is true that wisdom "spreads its branches out" over the entire world of thought and all the elements of life and activity. It covers good and evil, and truth and falsehood, so that from all of these the power of clarification and the light of life can be expanded. In fact, from matters of stupidity, matters of wisdom become known (Zohar, Vayakhel 47:2). Yet, in order to really understand matters, the feeling of the heart is often integral to the knowledge itself. In such a case, while he can imagine with his mind, he will not be able to grasp them until he also feels them with the heart.
The above creates a self-conflicting situation, which makes it incredibly hard for great people, who have elevated themselves properly in seeking truth and justice, to reach full wisdom. On the one hand, these people need to "arm themselves" with every area of knowledge in the world, including paths of evil, foreign influences, and inanity. These include feeling the negative elements, for only by internalizing them will one be able to rule over those elements and turn them into powers of goodness and divine light. On the other hand, if he feels them as they are, he runs the risk of actually falling into the "net" of the evil.
It is therefore necessary for the great man, who far exceeds a normal person, to first make special preparations in field of ethics, and develop a holy attribute of truth. Then he can delve into the very essence of matters until the depths of his heart and soul. If he succeeds at reaching this unique level, then even the lowly exposure to evil cannot harm him, but will enrich his recognition in a way that will enable him to contribute to the nation as a whole by teaching them how to avoid the pitfalls of the evil.
This ability, to reach the depths of all matters, whether good or bad, in a manner that encases all elements of his life, was a central goal of Shlomo in building his monarchal dynasty for all generations. That required him to go to the depths of even the most foreign worlds, including those from which a normal person would not be able to extricate himself. Shlomo, in contrast, had all the tools and the motivation to prepare himself. Because of that, the inclination toward evil developed in him only when he became older, by which time he had already filled himself with proper preparation to handle the challenge. That is when his wives turned his heart toward other gods. In such a case, the fact that his personal heart was not as complete as David’s, and in fact brought him to the brink of terrible sin, was actually an advantage in his quest to improve the nation. It was actually the tendency toward the sin that ensured that he would not sin because he internalized the fact that these sinful matters were full of depravity that can trap people. Only the greatest of all minds could succeed to follow such a dangerous path – to have his heart go toward sin but not to act in sin as a result.