The Jews gloried in the fact that they had God's Word unlike the rest of the world. But they did not live according to that Word; and they finally rejected the One about Whom God's Word had prophesied, when He came into their midst. It is possible for those of us who have understood the new covenant also to glory in a similar way and to deceive ourselves.
The following is an adaptation of Romans Chapter 2 (from the Living Bible) - adapted to the danger we face:
You may be saying, "How sad it is that so many Christians have not understood the new covenant that we have understood so clearly".
But wait! Examine your life honestly and see if it is better than the lives of those other Christians who have not understood the new covenant. Are you perhaps boasting only in a theoretical knowledge of the new covenant? Or have you actually entered into the new-covenant life?
We know that God is righteous and that there is no partiality with Him. Then, do you think He will condemn those other Christians for what they do, but overlook what you do, when you are doing the same things - just because you claim to be "a new-covenant Christian"?
God is actually being patient with you. He is giving you time to see your true condition in His sight. His kindness towards you is meant to lead you to judge yourself and to repent.
But you may be thinking that everything is all right with you, just because you not only understand the new covenant but can even explain it to others. You are then in an even greater danger of God's judgment - because you are not living according to the truths that you claim to be teaching others.
The day is coming when God will judge the secret lives of everyone - their inmost thoughts, attitudes and motives. He will then give each one exactly what his deeds deserved - eternal life to those who patiently did His will, and sought His glory and honour . And He will reject those who merely said that they knew the new covenant but walked in selfishness and spiritual pride and secret sin.
There will be eternal sorrow in that day, for both new-covenant Christians as well as for other Christians, if they had kept on sinning. But there will be glory and honour from God for those who obeyed him, whether they called themselves "new-covenant" Christians or any other type of Christians. There is no partiality with God.
The truth is that God will punish sin wherever it is found. He will punish even non-Christians for their sin, because even though they don't have a Bible, He has given them a conscience that tells them what is right and what is wrong. God's laws are written within them and their conscience either accuses them, or excuses them.
But God will punish new-covenant Christians even more, if they sin, because they knew His laws better than the others, but did not obey them. Salvation is not given to those who merely know what to do, but to those who actually do it. And God expects more from those to whom He has given more.
So you "new-covenant Christians" should not imagine that all is well between you and God just because you understand His truths. You may even be imagining that you are God's special friends, because you know what He wants; you know right from wrong and you have understood the terms of His new covenant clearly. You may also be imagining that you are beacon lights that can guide the simple-minded into the deeper truths of God.
But examine yourselves first. Perhaps you tell others not to steal - but have you ever stolen the credit for what you did not do? You say it is wrong to commit adultery-but do you lust after pretty girls? You say "Don't worship idols" - but do you make money your god?
Are you proud of knowing God's new covenant, but dishonouring Him by the way you live?
Knowing the new covenant is worth something only if you live according to it. But if you don't live according to it, then you are no better than anyone else. And if those other Christians (whom you look down on) obey God's laws, then God will reward them even if they did not understand the new covenant. They will be better off in the day of judgment, than you who knew so much about God and His promises but didn't live accordingly.
You are not a new-covenant Christian just because you belong to a new-covenant church or because you understand its truths. No. A real new-covenant Christian is one whose heart is always right with God because he judges himself and therefore lives an overcoming life every day. God is not looking for those who have their doctrines right, but for those who have allowed their hearts and minds to be changed by the Holy Spirit to actually walk in Jesus' footsteps daily. Only those who have that kind of change in their lives will get their praise from God in the final day.
He who has ears to hear let him hear.