"But seek ye first the kingdom of God, and his righteousness; and all these things shall be added unto you." Matthew 6: 33)
Jesus offers this powerful, intimidating, yet exhilarating exhortation to the Jewish multitudes, including His disciples, in the Sermon on the Mount.
Did he mention the Kingdom of Heaven prior to Matthew 6: 33?
"3Blessed are the poor in spirit: for theirs is the kingdom of heaven." (Matthew 5: 3)
and
"10Blessed are they which are persecuted for righteousness' sake: for theirs is the kingdom of heaven." (Matthew 5: 10)
Then later on, Jesus says:
"Whosoever therefore shall break one of these least commandments, and shall teach men so, he shall be called the least in the kingdom of heaven: but whosoever shall do and teach them, the same shall be called great in the kingdom of heaven.
"Seek ye first the Kingdom of God, and His righteousness."
We find the phrase Kingdom of God, not heaven.
What's going on here?
Jesus is emphasizing that this Kingdom is not a place which we get to through our efforts.
This is a kingdom which He gives to us through His Holy Spirit.
We receive this gift through grace. For the Jews of Jesus day, they believed that they could work their way to God. I believe that when Jesus specifically and directly stated "Kingdom of God", he heightened the offense for those beset with religious pretense.
We receive the Kingdom of God as a gift, as confirmed in the Gospel of Luke:
"Fear not, little flock; for it is your Father's good pleasure to give you the kingdom." (Luke 12: 32)
Jesus wants us to just be His sheep, to identify with God as our Father, and to receive the gift of His Kingdom.
Yet we can only receive the Kingdom as a gift of grace.