Here is an example.

I was at a sandwich shop the other day and there were two ladies that appeared to have just come from church. They took their time and caring only for their own wants and desires not caring about the next person in line. When they finally moved out of the way and then sat down and started to eat their sandwiches one of the women loudly voiced that their chips were stale. They proceed to insist that the only person behind the counter address their issues at that very moment. Not caring that she was busy with another customer at that moment.

This is an example of a lack of common courtesy.  

Per the Cambridge Dictionary:

common courtesy/decency – the basic level of politeness that you expect from someone.

This has another name. The golden rule:

Matthew 7:12 (NKJV) – Therefore, whatever you want men to do to you, do also to them, for this is the Law and the Prophets.

Ever notice how the golden rule is followed by these verses?

Matthew 7:13-27 (NKJV) – “Enter by the narrow gate; for wide is the gate and broad is the way that leads to destruction, and there are many who go in by it. Because narrow is the gate and difficult is the way which leads to life, and there are few who find it. “Beware of false prophets, who come to you in sheep’s clothing, but inwardly they are ravenous wolves. You will know them by their fruits. Do men gather grapes from thornbushes or figs from thistles? Even so, every good tree bears good fruit, but a bad tree bears bad fruit. A good tree cannot bear bad fruit, nor can a bad tree bear good fruit. Every tree that does not bear good fruit is cut down and thrown into the fire. Therefore by their fruits you will know them. “Not everyone who says to Me, ‘Lord, Lord,’ shall enter the kingdom of heaven, but he who does the will of My Father in heaven. Many will say to Me in that day, ‘Lord, Lord, have we not prophesied in Your name, cast out demons in Your name, and done many wonders in Your name?’ And then I will declare to them, ‘I never knew you; depart from Me, you who practice lawlessness!’ “Therefore whoever hears these sayings of Mine, and does them, I will liken him to a wise man who built his house on the rock: and the rain descended, the floods came, and the winds blew and beat on that house; and it did not fall, for it was founded on the rock. “But everyone who hears these sayings of Mine, and does not do them, will be like a foolish man who built his house on the sand: and the rain descended, the floods came, and the winds blew and beat on that house; and it fell. And great was its fall.”

The Word of God tells us to place all others before ourselves:

Romans 12:1-3 (NKJV) – I beseech you therefore, brethren, by the mercies of God, that you present your bodies a living sacrifice, holy, acceptable to God, which is your reasonable service. And do not be conformed to this world, but be transformed by the renewing of your mind, that you may prove what is that good and acceptable and perfect will of God. For I say, through the grace given to me, to everyone who is among you, not to think of himself more highly than he ought to think, but to think soberly, as God has dealt to each one a measure of faith.

When someone cares more about their stale chips over the needs of the person now in line being taken care of they deny this teaching. This leads us to this teaching from Christ Himself:

John 14:15-21 (NKJV) – “If you love Me, keep My commandments. And I will pray the Father, and He will give you another Helper, that He may abide with you forever— the Spirit of truth, whom the world cannot receive, because it neither sees Him nor knows Him; but you know Him, for He dwells with you and will be in you. I will not leave you orphans; I will come to you. “A little while longer and the world will see Me no more, but you will see Me. Because I live, you will live also. At that day you will know that I am in My Father, and you in Me, and I in you. He who has My commandments and keeps them, it is he who loves Me. And he who loves Me will be loved by My Father, and I will love him and manifest Myself to him.”

This teaching from Jesus Christ is then followed shortly after by these verses:

John 15:1-6 (NKJV) – “I am the true vine, and My Father is the vinedresser. Every branch in Me that does not bear fruit He takes away; and every branch that bears fruit He prunes, that it may bear more fruit. You are already clean because of the word which I have spoken to you. Abide in Me, and I in you. As the branch cannot bear fruit of itself, unless it abides in the vine, neither can you, unless you abide in Me. “I am the vine, you are the branches. He who abides in Me, and I in him, bears much fruit; for without Me you can do nothing. If anyone does not abide in Me, he is cast out as a branch and is withered; and they gather them and throw them into the fire, and they are burned.

Now let’s apply these teachings from the Word of God to this teaching about common courtesy:

So common courtesy is a commandment from Christ know as the golden rule. Those who ignore it are seen as thistles bearing fruit which is not able to be done. And those that ignore this teaching have built their home upon sand and are washed away.

This is reinforced further in the book of John. Where Christ makes it very clear that they must obey His commandments and if they do not, they bear no fruit. Those that bear no fruit are cast into the fires of damnation because they have no love for Jesus Christ.

Therefore, those that show no common courtesy have no love for Christ and are not welcome into heaven. PERIOD.