Even after the fearful and trembling men have left, the army gathered for battle remains too large [ביאור שטיינזלץ]. A second, much deeper filtering process is required to select the most worthy fighters. While the initial separation dealt with physical fear, this next phase is designed to examine the inner spiritual world of the remaining men. The fighters left behind do not necessarily share Gideon's pure intentions, as idol worshippers are still mixed among their ranks [מלבי״ם].
To resolve this, God commands that the men be brought down to the water at Ein Harod [ביאור שטיינזלץ]. God promises a process of purification, using a concept borrowed from metallurgy. Just as a silversmith melts down metal to separate pure silver from dross and waste, this test will separate the righteous from those who have sinned through idol worship. The trial at the water is specifically designed to reveal who is accustomed to dropping to their knees to worship the idol Baal. Additionally, a tradition suggests that during that era, some individuals worshipped their own reflections and shadows cast upon the river water [רד״ק]. Ultimately, the test will clarify exactly who possesses the complete faith and pure intentions needed to join Gideon in battle [מלבי״ם].
God outlines the results of this test by noting both who will go to battle and who will not. This dual phrasing carries practical and psychological meaning. On one level, it reveals that anyone who fails the test would not have lasted in the actual campaign. Lacking true faith, such a person would have immediately retreated the moment he faced the reality of war [מלבי״ם]. On another level, the wording addresses a very human concern. Gideon might have worried that sending fighters home against their will would spark anger, arguments, and deep resentment. God assures Gideon that the rejected fighters will return home of their own free will, without any hard feelings. In fact, these dismissed men were not angry at all; they gladly handed over their food provisions and horns to the select few who were chosen to stay and fight [אלשיך].