Yichud Hasheim via Kabbalas Malchus
…However great the variety presented to you both by Nature and by history and by your own life, you have nonetheless grasped the fact, which you must now lay to heart as vital, that all this is the doing of One God, through Whose will everything everywhere has been and is, and Whose will has directed everything that has happened and will happen to you; One God everywhere and in everything. Everything comes from this One God both in heaven and on earth, and everything therefore conforms to one design, is part of one all-wise plan.
But above all, the most vital lesson to lay to heart is the this One God is your God, and that you have acknowledged Him in order to live rightly. … for in everything and with everything you have been summoned to the service of the One God. Strive to reach this One, and be one in heart as your God is One.
— Rav S.R. Hirsch, Horeb, Ch. 2, “Unity of God"