The author says: After investigating after what is the most necessary of the cornerstones and fundamentals of our religion, we found that the wholehearted acceptance of the unity of G-d is the root and foundation of Judaism. It is the first of the gates of the Torah, and it differentiates between the believer and the heretic. It is the head and front of religious truth, and one who strays from it - will not be able to perform religious deeds and his faith will not endure.
בְּבֵאוּר אָפְנֵי חִיּוּב יִחוּד הָאֵל יִתְבָּרַךְ בְּלֵב שָׁלֵם. וְזֶה פֶּתַח הַשַּׁעַר:
אָמַר הַמְחַבֵּר כַּאֲשֶׁר חָקַרְנוּ עַל מָה שֶׁהַצֹּרֶךְ אֵלָיו יוֹתֵר מִפִּנּוֹת דָּתֵנוּ וְשָׁרָשֶׁיהָ מָצָאנוּ יִחוּד הָאֱלֹהִים בְּלֵב שָׁלֵם שָׁרְשָׁהּ וִיסוֹדָהּ שֶׁהוּא הַשַּׁעַר הָרִאשׁוֹן מִשַּׁעֲרֵי הַתּוֹרָה וּבַיִּחוּד יִפָּרֵד הַמַּאֲמִין מִן הַכּוֹפֵר וְהוּא רֹאשׁ אֲמִתַּת הַדָּת. וּמִי שֶׁנָּטָה מִמֶּנּוּ לֹא יִתָּכֵן לוֹ מַעֲשֶׂה וְלֹא תִּתְקַיֵּם לוֹ אֱמוּנָה.
The author says: After investigating after what is the most necessary of the cornerstones and fundamentals of our religion, we found that the wholehearted acceptance of the unity of G-d is the root and foundation of Judaism. It is the first of the gates of the Torah, and it differentiates between the believer and the heretic. It is the head and front of religious truth, and one who strays from it - will not be able to perform religious deeds and his faith will not endure.