Jesus said, "Do not think that I came to abolish the Law or the Prophets; I did not come to abolish but to fulfill. For truly I say to you, until heaven and earth pass away, not the smallest letter or stroke shall pass from the Law until all is accomplished. Whoever then annuls one of the least of these commandments, and teaches others to do the same, shall be called least in the kingdom of heaven; but whoever keeps and teaches them, he shall be called great in the kingdom of heaven." (Matthew 5:17-19).
When Jesus came, He fulfilled the laws of the Old Testament. But He also gave us new laws to live by, such as to love one another deeply.
Even though we are not saved by keeping the Law, we are expected to keep God's laws that are written in our hearts as Christians. True Christianity is not relying on Jesus to do everything for us and we do nothing. True Christianity is believing in Jesus. And as proof of our belief, to live as Christ teaches us to live.
Psalm 119 tells us that we need to live out our belief. The entire psalm praises God's laws as righteous. The psalmist desired to live by God's ways. In so doing, he was blessed.
If we look at the Bible as a whole and in a few sentences summarize what it tells us, we can see this:
God is righteous and mankind has sinned. God provides a way for mankind to come back to Him and to be forgiven and saved. Jesus died for us on the cross to take the penalty of our sins upon Himself. By His righteousness, we can live and have eternal life. God paves the way for us so that we can become righteous. We, on our part in our new life in Him, learn to do what's right, avoid what's evil, and bear fruit for the kingdom of God.
Psalm 119 is one of the parts in the Bible that teaches us to do what's right by keeping God's laws.
"You have ordained Your precepts,
That we should keep them diligently." (Psalm 119:4)
"Your word I have treasured in my heart,
That I may not sin against You." (Psalm 119:11)
If we love God, we would keep His commandments. Just as Jesus said, "He who has My commandments and keeps them is the one who loves Me; and he who loves Me will be loved by My Father, and I will love him and will disclose Myself to him." (John 14:21)
So Jesus promises to reveal Himself and be intimately close to those who keep His commandments.
What are Jesus' commandments? They include ones in the Old Testament such as: keep away from idolatry, do not use God's name in vain, do not murder, do not steal, honor parents, do not bear false witness, do not covet, etc. (Deuteronomy 5). They also include those in the New Testament such as: do not be angry with your brother (Matthew 5:22), do not lust (Matthew 5:28), do not make an oath (Matthew 5:34), love your enemies (Matthew 5:44), forgive one another (Matthew 6:14), treat others the same way you want to be treated (Luke 6:31), pursue faith, hope and love (1 Corinthians 13:13), etc.
Now if you read Psalm 119, does it make more sense?
Psalm 119:30-40 tells us that the psalmist has chosen the faithful way.
"I shall run the way of Your commandments,
For You will enlarge my heart." (Psalm 119:32)
He kept God's commandments because God enabled him to observe them (in other words, God enlarged his heart to perform them). We also can keep God's commandments because God has given us the Holy Spirit to help us do so.
The psalmist loved God's commandments "for Your ordinances are good" (Psalm 119:39).
We should also put our hearts into obeying God, asking God at the same time to help us obey His commandments.
"Give me understanding, that I may observe Your law
And keep it with all my heart." (Psalm 119:34)
This is God's will for us in Jesus Christ.
"…Walk in a manner worthy of the Lord, to please Him in all respects, bearing fruit in every good work and increasing in the knowledge of God..." (Colossians 1:10)