Rabbeinu Yonah lists denial of foundational beliefs among the gravest sins, and elaborates on the components of wholehearted אמונה — belief in God, Torah from Heaven, and reward and punishment.
וּמַעֲלוֹת הִתְבּוֹנֵן בִּגְדֻלַּת ה' שֶׁנֶּאֱמַר (דברים ד':ל"ט) וְיָדַעְתָּ הַיּוֹם וַהֲשֵׁבֹתָ אֶל לְבָבֶךָ כִּי ה' הוּא הָאֱלֹהִים, וְדָוִד אָמַר (תהילים י"ד:ב') ה' מִשָּׁמַיִם הִשְׁקִיף עַל בְּנֵי אָדָם לִרְאוֹת הֲיֵשׁ מַשְׂכִּיל דֹּרֵשׁ אֶת אֱלֹהִים וּמַעֲלוֹת זִכְרוֹן חֲסָדָיו וְהִתְבּוֹנֵן בָּהֶם שֶׁנֶּאֱמַר (דברים ח':ב') וְזָכַרְתָּ אֶת כָּל הַדֶּרֶךְ וְנֶאֱמַר (דברים ח':ה') וְיָדַעְתָּ עִם לְבָבֶךָ כִּי כַּאֲשֶׁר יְיַסֵּר אִישׁ אֶת בְּנוֹ יְיָ אֱלֹהֶיךָ מְיַסְּרֶךָּ. וְדָוִד אָמַר (תהילים ק"ז:מ"ג) וְיִתְבּוֹנְנוּ חַסְדֵי ה', וְנֶאֱמַר (תהילים כ"ו:ג') כִּי חַסְדְּךָ לְנֶגֶד עֵינָי. וּמַעֲלוֹת הַקְּדֻשָּׁה, שֶׁנֶּאֱמַר (ויקרא י"א:מ"ד) וְהִתְקַדִּשְׁתֶּם וִהְיִיתֶם קְדוֹשִׁים. וּמַעֲלוֹת הָעֲבוֹדָה, שֶׁנֶּאֱמַר (דברים י':כ') אוֹתוֹ תַעֲבֹד. וּמַעֲלוֹת הַיִּרְאָה, שֶׁנֶּאֱמַר (שם) אֶת ה' אֱלֹקֶיךָ תִּירָא. וּמַעֲלוֹת הָאַהֲבָה שֶׁנֶּאֱמַר (דברים ו':ה') וְאָהַבְתָּ אֵת ה' אֱלֹקֶיךָ וּמַעֲלוֹת הַדְּבֵקוּת שֶׁנֶּאֱמַר (דברים י':כ') וּבוֹ תִדְבָּק. לְכָל אַחַת מֵהֵנָּה כַּמָּה מַדְרֵגוֹת כַּאֲשֶׁר יִתְבָּאֵר בעז"ה.
“and you shall walk in His ways;” and the virtues of contemplation of the greatness of the Lord, as it is stated (Deuteronomy 4:39), “Know therefore this day and keep in mind that the Lord alone is God in heaven above and on earth below; there is no other,” and David said (Psalms 14:2), “The Lord looks down from heaven on mankind to find a man of understanding a man who seeks God”; and the virtues of remembrance of His kindnesses,” as it is stated (Deuteronomy 8:2), “Remember the whole way,” and it is [also] stated (Deuteronomy 8:6), “And you shall know that the Lord your God disciplines you just as a man disciplines his son,” and David said (Psalms 107:43), “he will contemplate the kindnesses of the Lord,” and said (Psalms 26:3) “For Your kindness is across from my eyes”; and the virtues of holiness, as it is stated (Leviticus 11:44), “and you shall sanctify yourselves and you shall be holy”; and the virtues of worship, as it is stated (Deuteronomy 10:20), “and He shall you worship;” and the virtues of fear, as it is stated (Deuteronomy 10:20), “And you shall fear the Lord”; and the virtues of love as it is stated (Deuteronomy 6:5), “And you shall love the Lord, your God;” and the virtues of clinging, as it is stated (Deuteronomy 10:20), ”to Him shall you cling.” There are several levels to each of these, as will be explained, with God’s help.