To the OP: I think that in most cases, this sort of question is based on an erroneous “understanding” as to what “the Kingdom of Heaven” means. We assume that “Kingdon of Heaven”=“The place you go after you die if you’ve prayed the sinners prayer and really, really meant it.”
A kingdom is a region in which a king reigns. If the King doesn’t reign in any man’s life, then that man is not ruled by the king’s authority. He hasn’t entered into the Kingdom of God. This would not, as I understand it, be as difficult a concept for a Jew of Jesus’ day. To me, a kingdom is a country with physical borders. To most of us, God’s kingdom is a sort of place…the place we think of as Heaven.
Jesus was sort of stating the obvious, but it still needed saying. “If you’re not obeying the King, then you’re not under his authority. You ought to be. He has a right to your obedience, but you’re not giving it, so you’re not in (subject to) the kingdom of God.
The hopeful part of this is that anyone, at any time, can repent and believe the gospel (good news) that Jesus Christ came to save sinners… from what? From their SINS. And sin consists in, basically, disobeying the commands of God. We are, through Him, saved from our sins. When anyone accepts this and stops sinning through the power of Christ, then he is in the Kingdom of God. He doesn’t have to pack up his suitcase and hop on a plane to get there. Just obey the King and you’re in the Kingdom right now. You don’t even have to die (physically). Obedience puts you there.