Jesus knew the tendency of human nature to think it can get to heaven by what it does. So how did the Master Teacher answer “the question of the ages”?
Rather than telling the inquiring law-expert, “Just believe in Me...” and letting it go at that, the Lord redirected his question: “What is written in the law? What is your reading of it?” The lawyer answered: “‘You shall love the LORD your God with all your heart, with all your soul, with all your strength, and with all your mind,’ and ‘your neighbor as yourself.’ ” And He said to him, “You have answered rightly; do this and you will live”’ (Luke 10:26-28).
The word “do” here means “continual action”—in other words, “keep on doing without stopping.” So, if one could always and constantly keep on loving God perfectly with all one’s heart and soul and strength and mind, and on top of that one’s neighbor as oneself, that person would indeed earn eternal life.
However, to slip up even once, that person is doomed, as James writes: “...For whoever shall keep the whole law, and yet stumble in one point, he is guilty of all...” (James 2:10). And St. Paul wrote: “For as many as are of the works of the law are under the curse; for it is written, “...Cursed is everyone who does not continue in all things which are written in the book of the law, to do them”’ (Galatians 3:10).
So as far as the question of the ages is concerned, the law of God answers clearly that the sinner cannot justify (save) himself. Yet when the lawyer tried to justify himself by asking “And who is my neighbor?” (v. 29), the Lord taught him and teaches all of us the parable of the Good Samaritan.
That parable ultimately points any and all inquirers to find eternal life (“justification” before God) only and alone through faith in the ultimate Good Samaritan, Jesus.
The Law is good; but since the Fall
Its holiness condemns us all;
It dooms us for our sin to die
And has no power to justify.
To Jesus we for refuge flee,
Who from the curse has set us free,
And humbly worship at His throne,
Saved by His grace through faith alone.
(The Lutheran Hymnal, 295:5-6)