Raissa Maritain notes in her writings that the New Testament covenant presents a higher standard than the Old Covenant. (Testament means Covenant, so I could write Old Covenant and New Covenant for Old and New Testament. One testifies to a covenant, even a marriage covenant, the hesed.)
That the New Covenant demands a higher level of the life of virtue simply follows the fact that now the graces bound by Original Sin flow through the Church to us in Christ, through His Passion, Death, and Resurrection.
We are called to greater holiness than the Hebrews in the desert of Sinai. We have grace, the sacraments, baptism for adoption as sons and daughter of God, confession for purification, Eucharist for endurance. We have the witnesses of 2,000 years of those who chose Christ, His Mother, His Church over sin and error. The sons of Aaron who perished only had to stop complaining, stop murmuring against God's chosen leader and obey him. They only had to become humble and leave prideful thoughts, leave the pursuit of power among the chosen ones.
Things have not changed, except we have more grace, more chances to turn to Christ and be saved.
Pray that those who have fallen away will respond to those graces the Lord gives them to return to Him.