The third introduction necessary to have proper bitachon: The person must single out G‑d as the only one he relies upon for those things regarding which he is obligated to rely on Him, and he does not partner any other being with Him to rely both on G‑d as well as on another being. For his trust in G‑d will be ruined when he partners another being with Him.

וְהַהַקְדָּמָה הַשְּׁלִישִׁית, שֶׁיְּיַחֵד אֱלֹהָיו בְּבִטְחוֹנוֹ בּוֹ בְּמַה שֶּׁהוּא חַיָּב לִבְטוֹחַ בּוֹ, וְאַל יְשַׁתֵּף זוּלָתוֹ עִמּוֹ וְיִבְטַח עָלָיו וְעַל אֶחָד מֵהַבְּרוּאִים, וְיִפָּסֵד בִּטְחוֹנוֹ בֵּאלֹהָיו בְּהִשְׁתַּתֵּף זוּלָתוֹ עִמּוֹ.

A person who relies on any being other than G‑d to help him, thinking that this other being may also sometimes help him and forgetting that G‑d is the only power in the world, cannot be considered a person who has bitachon.

You surely know that which is said regarding King Assa of Judah, who, despite his great piety, relied upon the doctors at the time when he was ill, as it is written (II Chronicles 16:12): And also when he was ill he did not seek the L-rd, only the doctors , and he was punished for this.

וּכְבָר יָדַעְתָּ מַה שֶּׁנֶּאֱמַר בְּאָסָא עִם חֲסִידוּתוֹ עֵת שֶׁסָּמַךְ עַל הָרוֹפְאִים, דִּכְתִיב (דברי הימים ב' טז, יב) "וְגַם בְּחָלְיוֹ לֹא דָרַשׁ אֶת ה' כִּי בָּרוֹפְאִים," וְנֶעֱנַשׁ עַל זֶה,

King Assa should have asked G‑d to heal him and then rely on Him to heal him either through the doctors or through whichever means He saw fit. Instead, the king relied solely on the doctors to heal him, for which he was punished.
Another verse that serves as proof that a person should not combine his bitachon in G‑d with another being:

Relied upon the doctors. King Assa erred in relying on physicians alone, and he failed to place his trust in G‑d, Who would provide the healing through the doctor ( Marpei Lanefesh ).

The Shulchan Aruch states: Permission has been granted to a doctor to heal, and it is a commandment upon him to heal. This is considered saving human lives ( pikuach nefesh ), and absent of his curing he is spilling blood ( Yoreh Dei’ah 336:1). The Taz (ibid. 1) explains the contradictory words permission and commandment to mean that a cure comes from G‑d through beseeching Him for mercy and health. G‑d, however, created the world in a way that His cure comes through the medium of a doctor. He has in a sense given a health professional His “permission” to heal. Now, the doctor is commanded to act on his G‑d-given ability to cure.

A man wrote to the Lubavitcher Rebbe that his wife was instructed by doctors to be hospitalized, but she was afraid of the hospital. The Rebbe replied: “You should explain to her, in words that she can appreciate (based on her current state of mind), that G‑d created the world, and it is He Who controls it. He controls both the Western Hemisphere, where I am situated, and the Eastern Hemisphere, where... you are situated. The verse states, He spoke, and it came into being; He commanded, and it endured (Psalms 33:9); thus, nothing takes place in the world without G‑d, and everything that He desires takes place. However, G‑d wants us to make a natural medium—things should happen in a natural way.

“When a Jew, be it a man or a woman, doesn’t feel well and a doctor must be summoned, this does not mean that the doctor is going to do whatever he feels like doing. What is really happening is that G‑d has chosen this doctor to be His emissary and to carry out His mission.

“When a person has bitachon, trusting without any doubt that G‑d conducts the world, he is then granted the privilege of seeing this with his physical eyes, too, at every single step… Therefore, when your wife goes to the hospital on the doctor’s orders, she is still under G‑d’s surveillance. And He will watch over her and see to it that things will work out in the way that is best for both her physical and mental health” ( Igrot Kodesh of the Lubavitcher Rebbe, vol. 3, p. 441).

Bottom line
When a person has bitachon, trusting without any doubt that G‑d conducts the world, he is then granted the privilege of seeing this with his physical eyes.

He was punished for this. He was not cured, because he lacked in his trust ( Ralbag, I Chronicles 16:12).

The verse says, Blessed is the man who relies on the L-rd, and the L-rd will be his support.

וְאָמַר הַכָּתוּב (ירמיה יז, ז) "בָּרוּךְ הַגֶּבֶר אֲשֶׁר יִבְטַח בַּה' וְהָיָה ה' מִבְטַחוֹ."

If the verse merely intended to extol the virtue of relying on G‑d, then it would have merely stated, Blessed is the man who relies on the L-rd. The second half of the verse, and the L-rd will be his support, implies that no other being is his support. This verse is cited as proof that true reliance on G‑d consists of reliance solely on Him and not on any other being in addition to Him.

It is a well-known matter that when someone appoints two people or more to perform a specific task, the appointment will fail. Each one relies on the other to do the task, and in the end it isn’t done properly.

וּמִן הַיָּדוּעַ כִּי מִי שֶׁיְּמַנֶּה מִבְּנֵי אָדָם שְׁנֵי מְמֻנִּים אוֹ יוֹתֵר לַעֲשׂוֹת דָּבָר, מִנּוּיוֹ מֻפְסָד,

All the more so does this apply to a person who relies on G‑d as well as on others—his reliance on G‑d will be demolished. This will be the main reason why the matter about which he relied on G‑d will be withheld from him.

כָּל שֶׁכֵּן מִי שֶׁבָּטַח עַל הָאֱלֹהִים וְזוּלָתוֹ, שֶׁיִּסָּתֵר בִּטְחוֹנוֹ וְיִהְיֶה זֶה הַסִּבָּה הַחֲזָקָה לְהִמָּנַע מִמֶּנּוּ מַה שֶּׁבָּטַח עָלָיו,

True bitachon is achieved when the person realizes that everything is from G‑d, and that He is the cause of everything. Any other things that appear to be the cause are in reality mere agents of G‑d that He uses to give people what they need.

All the more so does this apply to a person who relies on G‑d as well as on others—his reliance on G‑d will be demolished. Since he displays a lack of trust in G‑d’s ability, and he equates a mortal creation with G‑d, therefore, G‑d will withhold His help and allow him to see if the others that he has included in his trust can help him ( Tov Halevanon; Pat Lechem ).

Bottom line
Treat G‑d’s desires in the same manner that you treat your own.

As the verse says (Jeremiah 17:5): Cursed is the man who relies on man and places flesh as his support, and who removes his heart from the L-rd.

כְּמוֹ שֶׁאָמַר "אָרוּר הַגֶּבֶר אֲשֶׁר יִבְטַח בָּאָדָם וְשָׂם בָּשָׂר זְרוֹעוֹ וּמִן ה' יָסוּר לִבּוֹ" (ירמיה יז, ה).

A person who makes flesh his support and places his trust in other people will be cursed —i.e., he won’t receive the things he relied on them to provide for him.