What kind of test?
The story of the Akedah tells us that God tested Abraham.
The Hebrew uses a word that means more than a test in the sense of a school examination. The verb nisah means to be tried and tested through suffering.
In that sense the Akedah is not unique. Think for example of Job, the righteous man who lost one precious thing after another because God was testing his piety.
In both cases there are questions we ask, and in some way the questions are much the same.
In regard to Abraham, we wonder why the test had to be so severe. Test the patriarch, we want to say to God, by making his herds become unhealthy, his crops suffer blight or reducing his possessions.
Why go so far as to make him think his beloved son had to be killed?
In regard to Job, we can say likewise, “Reduce his bank balance, make his house collapse, send a hurricane to batter his daily life”. Why go so far as to attacking his health and his family?
The sages thought that only a real existential challenge would work. A serious test had to be serious.
They gave the example of a potter. If the potter wants to check the quality of his pots will he bother with the ones that he already doubts? The only way to test his pots is to test the really good ones.
That’s why God’s tests of His tzaddikim have to be really severe.