Why was the cross the greatest act of love?
The cross was the greatest act of love because Christ died for undeserving sinners, not for angelic beings or the righteous. It epitomized love, pouring out God's undeserved favor and mercy on sons of Adam, demonstrating grace rather than law and showing that He loved humanity even while they were still sinners.
Les explains that grace is God's attribute capable of pouring out undeserved favor and mercy on sinners like us. The cross was love epitomized, as great an act of love as has ever been done, because He loved us as sinners—undeserving sons of Adam—not as angelic beings. Paul therefore refuses to frustrate the grace of God by legalism, because if righteousness came by the law, then Christ is dead in vain.