Rightly divide the Word of Truth, soldiers in Christ! (2 Timothy 2: 15)
Jesus did not deliver these words to us or to our day, under the New Covenant.
Matthew Five lays the groundwork for the audience whom Jesus is addressing:
"And seeing the multitudes, he went up into a mountain: and when he was set, his disciples came unto him: 2And he opened his mouth, and taught them, saying," (Matthew 5: 1-2)
Jesus is instructing specifically His disciples.
Today, we are no longer disciples, but sons of the Living God through Jesus (John 1:12; Romans 8:15) There is no reference to discipleship after the Book of Acts, either.
So, Jesus tells His disciples to "seek first the kingdom of God and His righteousness."
This standard should put everyone to shame and despair. None of us can seek His Kingdom, for His ways are not our ways (Isaiah 55: 8). Besides, Heaven is His throne, and earth is His footstool , and He wants to dwell in us (Isaiah 66: 1-2).
His righteousness is not something that we can do or achieve on our own, either.
It is a gift which we receive and keep receiving:
"For if by one man's offence death reigned by one; much more they which receive abundance of grace and of the gift of righteousness shall reign in life by one, Jesus Christ." (Romans 5: 17)
and today, we are already citizens of His Kingdom:
"but our citizenship is in heaven, from which we also eagerly wait for a Savior, the Lord Jesus Christ." (Philippians 3: 20)
"Who hath delivered us from the power of darkness, and hath translated usinto the kingdom of his dear Son:" (Colossians 1: 13)
What they had to achieve under law, we receive today under grace!