Abraham’s greatest challenge came 37 years after his son Isaac was born. G‑d commanded him to sacrifice Isaac..
וַיֹּאמֶר קַח נָא אֶת בִּנְךָ אֶת יְחִידְךָ אֲשֶׁר אָהַבְתָּ אֶת יִצְחָק וְלֶךְ לְךָ אֶל אֶרֶץ הַמֹּרִיָּה וְהַעֲלֵהוּ שָׁם לְעֹלָה עַל אַחַד הֶהָרִים אֲשֶׁר אֹמַר אֵלֶיךָ: (בראשית כב:ב)
[G‑d] said [to Abraham], “Please Take your son, your only one, the one you love, Isaac, and go to the land of Moriah. Take him up there as an ascent-offering, on one of the mountains that I will designate to you.” Genesis 22:2

The primary aspect of this test was not the self-sacrifice it entailed but the challenge it posed to Abraham’s implicit faith in G‑d: G‑d had promised Abraham that Isaac would be the one to perpetuate his legacy; now G‑d was commanding Abraham to sacrifice Isaac, in seeming contradiction to His very own word. Yet Abraham unquestioningly carried out G‑d’s command.

G‑d tests us in order to bring our hidden soul-powers to the fore. In fact, life in general is such a test. Before it descended into this world, the soul related to G‑d within the limits of reason; the soul never experienced a love for Him that transcends reason. But once the soul is encased in a physical body, which is by nature antagonistic to spirituality, it must summon its innermost strength to remain faithful to G‑d despite life’s daily trials and tribulations. With this newfound strength, the soul comes to understand and appreciate G‑d in a much more profound and intimate way than it ever could have before descending into this world.1