Akeidas Yitzchok was the ultimate test of Avraham Avinu, after which Hashem proclaimed that now he has true yiras elokim (fear of G-d). Until this point, Avraham understood what he was doing in his service of Hashem, and continued coming close to him with ahavah (love). When Hashem asks him to sacrifice his son, it is the antithesis of everything he knows and understands about Hashem. By following Hashem's orders, he demonstrates a complete subservience to Hashem's requests, regardless of whether or not he understands. This adds the critical element of yirah (fear) to his relationship with Hashem, instead of an all-encompassing love.