"But seek ye first the kingdom of God, and his righteousness; and all these things shall be added unto you." (Matthew 6: 33)
This commandment was never given to the Gentiles, and therefore it does not belong to you and me today, whether Jew or Gentile, living under the New Covenant.
Jesus was teaching his disciples the perfection of God's law, which is totally beyond any one of us to keep:
"1And 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)
Notice that even though there were multitudes who went up the mountain, the disciples came to him, and he taught them, i.e. the disciples.
"20For I say unto you, That except your righteousness shall exceed the righteousness of the scribes and Pharisees, ye shall in no case enter into the kingdom of heaven." (Matthew 5: 20)
"48Be ye therefore perfect, even as your Father which is in heaven is perfect." (Matthew 5: 48)
Isaiah was crystal clear about our righteousness works:
"But we are all as an unclean thing, and all our righteousnesses are as filthy rags; and we all do fade as a leaf; and our iniquities, like the wind, have taken us away." (Isaiah 64: 6)
No one can be perfect, for no one is good, no not one (Psalm 14: 3)
So, how do we get His righteousness? It is a gift which is given to us:
"In righteousness shalt thou be established: thou shalt be far from oppression; for thou shalt not fear: and from terror; for it shall not come near thee." (Isaiah 54: 14)
"No weapon that is formed against thee shall prosper; and every tongue that shall rise against thee in judgment thou shalt condemn. This is the heritage of the servants of the LORD, and their righteousness is of me, saith the LORD." (Isaiah 54: 17)
We are established in righteousness; it is not something we establish in ourselves. This righteousness comes from God our Father, not from anything that we do.
How did God accomplish this work? Through His Son, Jesus!
"For he hath made him to be sin for us, who knew no sin; that we might be made the righteousness of God in him." (2 Corinthians 5: 21)
"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)
Because of Jesus and His Work at the Cross, we are established in righteousness, destined to reign in life as we receive and keep receiving this wonderful gift.
Under law, Jesus had to instruct self-righteousness disciples to seek something which our efforts can never attain. Because of the grace of God, we recognize the full revelation, that righteousness is the gift of God, apart from works, fulfilled for us in the Finished Work of Jesus!