I shall tackle this question with a systematic approach. All verses used are in KJV.
Before that, though, let’s talk about Luke 24:39,
Behold my hands and my feet, that it is I myself: handle me, and see; for a spirit hath not flesh and bones, as ye see me have.
In this verse, Jesus is resurrected in flesh and blood like an actual human being. He is telling His disciples that He is neither a ghost nor some demon spirit conjured by Satan to fool them, but He is Himself, literally, in the flesh. Flesh and blood in this scenario does not refer to sins of the flesh or worldly desires, it must be taken in the literal context.
Below will be the systematic approach on understanding who are those “flesh and blood” that cannot enter the kingdom of heaven. Firstly, we ask ourselves,
Who can inherit the kingdom of heaven?
Definitely not Jesus! He is the King of kings and the Lord of lords, how can a king inherit something that is of his possession? Much less the Lord Himself? Jesus did not come to earth as the Son of God to inherit the kingdom of heaven as the Son of man, He came down to proclaim the Word of God so that we, may.
Matthew 4: 17
From that time Jesus began to preach, and to say, Repent: for the kingdom of heaven is at hand.
Mark 1: 14-15
14 Now after that John was put in prison, Jesus came into Galilee, preaching the gospel of the kingdom of God,
15 And saying, The time is fulfilled, and the kingdom of God is at hand: repent ye, and believe the gospel.
What does the Bible say about those who can enter the kingdom of heaven?
Anyone who receives Christ can enter the kingdom of heaven.
1 John 5:11-12
11 And this is the record, that God hath given to us eternal life, and this life is in his Son.
12 He that hath the Son hath life; and he that hath not the Son of God hath not life.
John 3: 16
For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.
Okay, that sounds easy enough. So, what is the Bible’s definition of belief/faith?
An action faith manifested through good works!
18 Yea, a man may say, Thou hast faith, and I have works: shew me thy faith without thy works, and I will shew thee my faith by my works.
19 Thou believest that there is one God; thou doest well: the devils also believe, and tremble.
20 But wilt thou know, O vain man, that faith without works is dead?
21 Was not Abraham our father justified by works, when he had offered Isaac his son upon the altar?
22 Seest thou how faith wrought with his works, and by works was faith made perfect?
23 And the scripture was fulfilled which saith, Abraham believed God, and it was imputed unto him for righteousness: and he was called the Friend of God.
Believe, in the biblical context, is not just the mental capacity of admitting the existence of someone or events, it is your actions that defend (justify) your faith. For if one believes in Christ, one would follow Him and His righteous way. Just like in school, if you believe the mathematical formula is the right one, you would use it and not use another! How can you say you believe it is the right one yet refuse to use it? It would show that you didn't believe in the first place or didn't believe enough.
Accept Jesus Christ as Son of God and Saviour, repent of your sins and be righteous in all your ways and works. Through you shall God's glory be revealed to those who have not yet receive Christ because people can see your works and may make the decision to come to Christ! The second commandment is to love your neighbours as yourself. If you love your neighbours, you would want them to be saved through Jesus Christ as well.
When a christian is righteous, they will also keep God’s commandments.
Since the righteous can enter the kingdom of heaven, it means the unrighteous can’t. What does the Bible say about unrighteous people?
1 Corinthians 6:9-11
9 Know ye not that the unrighteous shall not inherit the kingdom of God? Be not deceived: neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor abusers of themselves with mankind,
10 Nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners, shall inherit the kingdom of God.
11 And such were some of you: but ye are washed, but ye are sanctified, but ye are justified in the name of the Lord Jesus, and by the Spirit of our God.
8 But the fearful, and unbelieving, and the abominable, and murderers, and whoremongers, and sorcerers, and idolaters, and all liars, shall have their part in the lake which burneth with fire and brimstone: which is the second death.
Basically, all the bad people who refuse to repent of their sins and continue sinning, cannot enter the kingdom of heaven.
How do you connect the unrighteous people to the "flesh and blood" that cannot enter the kingdom of heaven?
Here is where we come to the phrase "sins of the flesh" and "worldly desires". Evident in various verses!
1 John 2:15-17
15 Love not the world, neither the things that are in the world. If any man love the world, the love of the Father is not in him.
16 For all that is in the world, the lust of the flesh, and the lust of the eyes, and the pride of life, is not of the Father, but is of the world.
17 And the world passeth away, and the lust thereof: but he that doeth the will of God abideth for ever.
Love of the world = Worldly desires = Anything that is not the Will of God for the believer.
Lust of the flesh = Sexual immorality = sin, sin, sin!
19 Now the works of the flesh are manifest, which are these; Adultery, fornication, uncleanness, lasciviousness,
20 Idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, heresies,
21 Envyings, murders, drunkenness, revellings, and such like: of the which I tell you before, as I have also told you in time past, that they which do such things shall not inherit the kingdom of God.
HOLD UP. Does this not sound just like the sins of the unrighteous people from other verses?
Yes! The flesh manifests many sins! Lust, envy, pride, wealth, the list goes on! Hence, the flesh and blood that cannot enter the kingdom of heaven does not actually mean that as long as we are in our flesh, we cannot enter even though we've accepted Christ. Flesh and blood that cannot enter the kingdom of heaven is the sins of the flesh and the worldly desires that is not of the Will of God. So, don't worry! Just 'cause we are human, we can still inherit the kingdom of God!
Besides, have you heard of the phrase, the Spirit wars against the flesh?
It is derived from Galatians 5:17
For the flesh lusteth against the Spirit, and the Spirit against the flesh: and these are contrary the one to the other: so that ye cannot do the things that ye would.
What are the things that you would do? But cannot because the Holy Spirit is strong in you? For example, before you were christian, maybe you would throw a profanity here and there when you were extremely angry. However, after you became a christian and began to follow Christ, you toned down or completely did away with the swearing. This is part of the good works - when people see the change in you, they see the glory of God in you because you can now glorify His Name by saying, "I believe in God and Jesus, the Lord has changed me and I am starting to control myself now." This is known also as the works of faith and called the testimony of the Word of God. Are these words familiar to you? They should!
How can one say he believes in Jesus yet swears up and down whenever he wants? This is because the flesh is strong! When the flesh is strong, it manifest many nasty things. Swearing is one of them, or someone who is quick to anger, or rapist, adultery, many many sins, I dare say ALL SINS, come from the flesh. And these fleshly sins will deny the "believer" the kingdom of heaven.
I end this long wall of text with relevant verses from Proverbs 15, anything in brackets are my own interpretation.
1 A soft answer turneth away wrath: but grievous words stir up anger.
3 The eyes of the Lord are in every place, beholding the evil and the good.
8 The sacrifice of the wicked (even when he accepts Christ) is an abomination to the Lord: but the prayer of the upright is his delight.
9 The way of the wicked is an abomination unto the Lord: but he (The Lord) loveth him that followeth after righteousness (The Way).
10 Correction is grievous unto him that forsaketh the way: and he that hateth reproof shall die. (Hate God's discipline or commandments)
16 Better is little with the fear of the Lord than great treasure (wealth, desires) and trouble therewith.
17 Better is a dinner of herbs (simple needs) where love is, than a stalled ox (feasting) and hatred therewith.
18 A wrathful man stirreth up strife: but he that is slow to anger appeaseth strife.
19 The way of the slothful man is as an hedge of thorns: but the way of the righteous is made plain. (in the sight of the Lord, God recognises good works done in faith)
20 A wise son maketh a glad father: but a foolish man despiseth his mother.
21 Folly is joy to him that is destitute of wisdom: but a man of understanding walketh uprightly. (If you really believe, you would follow God's ways)
24 The way of life is above to the wise, that he may depart from hell (don't sin) beneath.
25 The Lord will destroy the house of the proud (no kingdom of heaven for pride, one of the seven deadly sins!): but he will establish the border of the widow.
26 The thoughts of the wicked are an abomination to the Lord: but the words of the pure are pleasant words.
29 The Lord is far from the wicked (no kingdom of heaven for them either): but he heareth the prayer of the righteous.
33 The fear of the Lord is the instruction of wisdom; and before honour is humility. (humble yourselves and repent)