Note 2 at Mt 19:17: Why did Jesus tell this man to keep the commandments? Many scriptures make it clear that no one can be justified in the sight of God by keeping the commandments (Hab 2:4; Ro 3:20, 10:3-13, 11:6; Ga 2:16, 3:1-3, 7-14; Eph 2:8-9; and Tit 3:4-7). Jesus knew this. However, this rich young ruler didn't know this. He was under the deception that there was some good thing he could do (Mt 19:16) that could purchase him eternal life (see note 3 at Joh 6:28).

The commandments (or laws) were never given for the purpose of producing salvation. Rather, their intent was to show mankind their exceeding sinfulness (Ro 3:20 and 7:13), so they would despair of trying to save themselves and would trust in a Savior (Ga 3:21-25). This was exactly what Jesus used the commandments for here. This rich young ruler either needed to keep all the Law perfectly or he needed a Savior. Jesus desired to turn this man from trusting in himself by showing him God's perfect standard, which no one could keep.