Quite likely, when your brain processed the phrase “this one is the greatest in the kingdom of heaven,” it placed the idea in the present tense. Also likely, I expect, when your brain processed the phrase “you will not enter the kingdom of heaven,” it placed the idea in the future tense, probably even aligning the meaning with “going to heaven when you die.”
I believe that Roman Catholic Mythology has influenced the shift from what Jesus intended, which was that both ideas were present tense in His mind. Entering the kingdom means becoming subject to the King. That is today and every day until there is no day or night – just Christ being the Light of the City of God.