Summary: No matter your status, as long as you are called by His name or you are a member of His vineyard, Matt 7:21-27 calls you a fool if you meddle your will with the Father's will or you have retained your old identity in following Him.
In the part of the world where I came from, it is insulting to direct such a question at an individual. The truth is that our response to any statement goes beyond the surface meaning. And in this case, we believe that before you could direct such a question at an individual, you have already concluded that the person concerned is a fool. You are only trying to confirm if the individual is aware of his/her foolishness. Be that as it may, I would not want you to take the question as my people would. Please, do not be offended by the tittle so that you don't throw the baby away with the bathing water. You are wondering what that means. Aren't you? It means that you should not allow the question to get at you so much that you give way for anger to becloud your sense of judgment.
Anyway, my job here is very simple and I am sure many of us would have at one time or the other used this parable of Jesus in a sermon, exhortation, talk or even prayers. In Matthew 7:21-27, Jesus Christ while trying to elucidate on the fact that it is not everybody who accepted Him as his/her Lord and Saviour nor everybody who proclaims Him as Lord nor every pastor/apostle/prophet/evangelist/teacher that will enter heaven gave an illustration that presents those that will not go to heaven though they believed on Him or did great works in His name as foolish. Then I ask, "Are you foolish?"
In Rev 3:14-22, Jesus passed like verdict on the Laodicean Church. They were well recognized as Christians but the only problem with them was that they were diplomatic with God. They were trying to sit on the fence. They knew God and His stand concerning daily Christian living but they were trying to cut corners. Meanwhile our God is not just all-knowing, He is a true Judge and Witness. You do not need to put in any effort to convince Him that what you have done is right, He already has a standard and that cannot be breached. It is therefore foolish for anyone to think that he/she can maintain double standard with Him.
In Galatians 6:7-8, we were enjoined not to deceive ourselves by trying to play games with God. He is not a man that can be mocked. His weighing balance does not and cannot malfunction and His judgment is always right and true. He does not rob Peter to pay Paul. The reason, with Him, everyone reaps with the same measure he/she sows. He therefore considers anyone who thinks he/she can fool Him a fool.
In Matthew 16:24-26, Jesus Christ informed the disciples that whoever would come after Him must first of all deny himself and then carry his cross before he/she could be qualified to follow Him. And the only people who will not lose their lives after following Him are those who would not have an alternative plan for themselves. It is nothing short of foolishness if you gain the whole world (which is not possible) and lose your soul or think you can give anything in exchange for your soul.
Just as the righteousness of man is not more than a filthy rag before God, 1Cor 3:19 describes the wisdom of this world as foolishness before God. And having accepted Jesus Christ who is the wisdom of God and got liberated from such wanton folly of the world, it is pitiable to have you think that you can neglect His ways for your old ways or marry the two ways in order to achieve a positive end with Him. The will of the Father is enshrined in the words of the Son. And it is these words that Psalm19:7-11describes as being perfect in converting the soul, sure in making wise the foolish, right in rejoicing the heart, pure in enlightening the eyes, more desirable than fine gold, sweeter than honey, a guide to believers and the only source of great reward if well kept. It is therefore foolishness to think that you can enjoy these benefits without inclining your heart to His words.