From: Muslim website
Kufr (unbelief) in the linguistic sense: “It is covering something and veiling it. And all of what covers something, then it has done kufr to it. And from this, the farmer can be called a kaafir, for he covers the seed with the dirt. Likewise, Allah I has said,
كمثل غيث أعجب الكفار نباته
‘The likeness of vegetation after rain, thereof the growth is pleasing to whoever (kuffar) that planted it.’
“This means it is pleasing to the tiller, and from that the tiller is named to be a kaafir, for he has veiled the favour of Allah I. Al Azhari says,
‘And His favours are his proven signs of His Tawhid. And the favours, which the kaafir has veiled, are the signs that enable the people to make the distinction that the Creator is One without partner. And likewise, he sent the Messengers with miraculous signs, books and abundant obvious proofs as a favour from Him.
‘Thus whoever is not truthful to the favour, and he rejects it, then he has become a kaafir (showed unbelief) in the favour of Allah, meaning that he has covered and veiled the favour from himself.’ ”
Shari`a definition of kufr: It is the complete decrease of imaan and it is the opposite of imaan. It is the unbelief in Allah, Mighty and Majestic, and His favours and the kufr here has two matters, kufrun akbar (major kufr) and kufrun asghar (minor kufr). In this case, the kufr is what the person is doing by covering the bounty and favour of Allah I from himself or others.
KUFRUN AKBAR (major kufr) In general, it is disbelief in Allah I, His Angels, His Messengers, His Books, The Day of Judgment and Divine Decree and Pre-ordainment, the good and the bad of it, in total or in any of these pillars. This also includes showing what is the opposite of the belief, for example by statements, actions and expressed beliefs of kufr that nullify imaan. Also, if he is not practicing the pillars of Islam that Allah I made as an apparent sign for a person to preserve his blood, honour and money, then this as well leads to apostasy. In our time, many people think that kufr is only to belie with the tongue, but this is not the belief of Ahl us-Sunna. There are many types of kufr that Allah mentioned in the Qur’an and the Sunna, but there are six in particular that constitute the major kufr.
a. Juhud. This is the denial of the signs of Allah I, whether it is a prophet, a miracle, an angel or a book brought by that prophet.
It has three forms,
1. Juhud alQalb (the denial of the heart). This is major kufr and happens to the people who have no link between their heart and their mind.
2. Juhud alLisaan, which is denying of the signs with the mouth, although the heart is satisfied with the truth. This denial is also called the juhud of arrogance and self-oppression (zulm). This is the most well known juhud for the evil tyrants. The Pharaoh of Egypt and his army remains one of history’s greatest examples of Juhud al-Lisaan,
فلما جاءتهم آياتنا مبصرة قالوا هذا سحر مبين و جحدوا بها و استيقنتها أنفسهم ظلماً و علواً فانظر كيف كان عاقبة المفسدين
“Whereas when Our signs came to them, clear to see with insight, they said, ‘This is clear and obvious magic. And they denied (juhud) them arrogantly and without reason, although they were convinced of it in themselves. So see what was the penalty of the corrupt ones. ’”
3. Juhud al`Amal, that being the juhud in action. This consists of the person not denying by the tongue, but denying by his action. This happens when he announces something opposite to what he has been told by Allah I, without contesting it directly. It is also when he acts opposite to the order deliberately, without contesting with the mouth.
The example for this type would be the individual that is a ruler and knows that the penalty for adultery in Islam is the death penalty. However, rather than implement the judgment of Allah I, he makes a rule that those who commit adultery are to be jailed only. Although he never denied the law per se, he has denied it simply by acting opposite to the order deliberately.
b. Takdhib. This matter here is to belie or to deny something from the words, signs or promise of Allah I, for example, the Judgment Day. There are three types,
1. Takdhib alQalb, where the heart rejects the truth.
2. Takdhib alLisaan, which has two forms,
a. This is to directly contest by saying, “Allah I said this, but that can’t be right.”
b. This is the takdhib of hinting about. An example of this is if someone were to say, “Allah I said X.” You then reply by saying, “No, Allah I said Y.” The Mushrikun (pagans) said the same thing,
سيقول الذين أشركوا لو شاء الله ما أشركنا و لا اباؤنا و لا حرمنا من شيء كذلك كذب الذين من قبلهم حتى ذاقوا بأسنا
“Those who make shirk (polytheism) will say, ‘If Allah willed, we would not have associated partners, neither ourselves nor our fathers, and we would not have made haraam anything.’ Likewise, those from before them lied until they tasted punishment.”
و قال الذين أشركوا لو شاء الله ما عبدنا من دونه من شيء نحن و لا اباؤنا و لا حرمنا من دونه من شيء. كذلك فعل الذين من قبلهم فهل على الرسل إلا البلاغ المبين
“And those who make shirk (polytheism) say, ‘If Allah had willed, we would have never worshipped anything besides Him, neither us nor our fathers. Nor would we have made haraam besides that anything.’ Likewise, those from before them did, so is it on the Messengers to give anything except the clear message?”
3. Takdhib al`Amal, a person is doing an action that shows that he is denying. This example can be acted out with the following verses from the Qur’an,
أ رأيت الذي يكذب بالدين فذلك الذي يدع اليتيم و لا يحض على طعام المسكين
“Have you not seen the one who denied (takdhib) the religion? Then that is who was hard on the orphans and did not hasten to feed the poor. ”
These simple three verses here refute the false belief that you have to do major kufr in speech before you can leave the religion due to major kufr. These verses here show that this is not the case. The verse doesn’t read, “Have you not heard” nor “Did he not deny with speech?” Of equal weight as well is the one who marries his mother. This Muslim who goes and commits this act exits from the religion of Islam and becomes a kaafir. He did not have to say anything in order to leave from the religion. The actions that he advertised showed that he was denying the rights of Allah I. The verses below can fit into the general category of Takdhib,
و ما أدراك ما يوم الفصل ويل يومئذ للمكذبين ألم نهلك الأولين
“And what will cause you to know what is the Day of Sorting (Judgment)? Woe be to the deniers (of the Day of Sorting) (takdhib). Did We not destroy those before?”
4. Istikbaar. This kufr is actually being proud and haughty towards the truth and expressing arrogance when the signs of Allah I are presented,
إذ قال ربك للملائكة إني خالق بشراً من طين فإذا سويته و نفخت فيه من روحي فقعوا له ساجدين فسجد الملائكة كلهم أجمعون إلا إبليس استكبر و كان من الكافرين
“And when your Lord said, ‘I am going to create a human being from clay. So when I have fashioned and shaped him and breathed into him his soul created by Me, then you are to prostrate to him.’ So all of the angels prostrated, except Iblis. He was proud (istikbaar) and became a kaafir.”
5. Istihzaa’. This sin is the mocking or making jest of any of the signs of Allah I. This could be in jesting about the religion of Islam, or it could be mocking something in the religion, like the veil over the face that Allah I made compulsory on the Muslim women, the verses from the Qur’an or statements and actions from the Prophet r. Allah I mentions the fate of such a people,
قل أ بالله و آياته و رسوله كنتم تستهزئون لا تعتذروا قد كفرتم بعد إيمانكم إن نعف عن طائفة منكم نعذبكم طائفة بأنهم كانوا مجرمين
“Say, ‘Was it in Allah, His verses or His Messenger that you were mocking (istihzaa’)? Make no excuse about it, you have disbelieved (become kuffar) after you had imaan. If we pardon a some of you, surely we will punish the others, for they were the criminals.’ ”
6. I`raad. This aspect is turning away from and fleeing from whatever admonition and signs that Allah I gives to guide someone from the darkness to the light. These people are mentioned in the following manner,
فما لهم عن التذكرة معرضين كأنهم حمر مستنفرة فرت من قسورة بل يريد كل امرئ منهم أن يؤتى صحفاً منشرة كلا بل لا يخافون الآخرة
“Then what is wrong with them that they flee (I`raad) from the remembrance (the Qur’an) as if they were donkeys fleeing from a lion? On the contrary, every one of them wants that he should be given pages spread out (of the revelation). By no means! On the contrary, they do not fear the Hereafter.”
7. `Inaad. This type of kufr is to be stubborn against Allah I and His signs and to insist on keeping others or oneself in kufr. Allah I has said,
القيا في جهنم كل كفار عنيد مناع للخير معتد مريب الذي جعل مع الله إلهاً أخر فألقياه في العذاب الشديد
“I will put in the Hellfire every stubborn (`inaad) kaafir, hindering from the good and increasing in evil and doubt, who made with Allah another god. Thus I will put him in the most severe punishment.”
8. Istibdaal. This is the kufr of replacing the Shari`a, and it has three manifestations, as has been listed by al `Allamah Ibrahim ibn Muhammad ibn Ibrahim,
i. Replacing the law of Allah I with man made laws. This is where someone actually legislates a new law and attributes it to the Shari`a or he makes his own fabricated Shari`a. This example is spoken of below,
أم لهم شركآء شرعوا لهم من الدين ما لم يأذن به الله و لو لا كلمة الفصل لقضى بينهم و إن الظالمين لهم عذاب اليم
“Or do they have partners for them legislating a religion that Allah did not give permission for at all. Had it not been for the word of decision and decree, the matter between them would have been judged. And truly, for the oppressors is a torturous punishment.”
Allah I also asked those who want to legislate or even head for the false legislators,
أفحكم الجاهلية يبغون و من أحسن الله حكماً لقوم يوقنون
‘Is it the legislation of the Jaahiliyyah (Days of Ignorance) that they seek? And who is better in judgment than Allah for a people who are certain?!’
ii. Denying the law of Allah I without renouncing them. This is the equivalent of someone saying, “This particular law doesn’t suit this time period, but these other laws are still satisfactory.”
أ فتؤمنون ببعض الكتاب و تكفرون ببعض فما جزاء من يفعل ذلك منكم إلا خزى في الحياة الدنيا و يوم القيامة يردون إلى أشد العذاب و ما الله بغافل عما تعملون أولئك الذين اشتروا الحياة الدنيا بالآخرة فلا يخفف عنهم العذاب ولا هم ينصرون
“Is it you believe in a part of the book and you disbelieve in another part?! Then what is the reward for the one who does that except disgrace in this life and on the Day of Judgment? They will be subjected to a worse punishment and Allah Almighty’s not unmindful of what you do. They are those who have purchased this life with the Hereafter. The punishment will not be lightened for them nor will they have any help.” 
Essentially, this is disbelief in one law while believing in another.
iii. Denying the law of Allah I and renouncing them. This is when someone denies the law of Allah and fails to judge by what Allah sent down all of the time.
This third type of Istibdaal is played out in the verses where Allah I says,
و من لم يحكم بما أنزل الله فاؤلئك هم الكافرون
“And whoever does not judge by what Allah sent down, then they are Kaafirun (unbelievers).”
و من لم يحكم بما أنزل الله فاؤلئك هم الظالمون
“And whoever does not judge by what Allah sent down, then they are Zaalimun (oppressors).”
و من لم يحكم بما أنزل الله فاؤلئك هم الفاسقون
“And whoever does not judge by what Allah sent down, then they are Faasiqun (rebellious sinners).”
Looking at the evidence in front of us, if someone is ruling by the Shari`a and he should refuse to hand out a Shari`a punishment in one instance due to the fact that the guilty party is his close friend, family member, etc., this is what we call the kufr duna kufr (a kufr less than kufr), a minor kufr.
But when the person actually changes the rule, not just for that one person, but for all times to come, this is a major kufr. But if he goes and even invents a stipulation in the existing Shari`a law, this is a kufr fawqa kufr (kufr above kufr), a major kufr without doubt.
و من لم يحكم بما أنزل الله فأولئك هم الكافرون
“And whoever does not judge by what Allah sent down, then they are Kaafirun (unbelievers).”
This is the completely naked and obvious kufr that is obnoxious and grotesque. This disregard for the Shari`a earns nothing but the wrath of Allah I, and it should be something that the sincere believers stay far away from.