One of the things necessary for the salik in all his worships, especially in the salat, which is at the head of all worships and has a position of comprehensiveness [jami'iyat] , is submissiveness [khushu]. It is, in fact, a complete submission [khudu'] mixed with love or fear. It is the result of comprehending the Greatness, Power and Majesty of Beauty and Glory. The detail of this generality is that the hearts of the people of suluk are different according to their disposition and nature:
Some of the hearts are amorous and of the manifestations of Beauty [jamal] and, driven by their nature, they are attracted to the Beauty of the Beloved. When, in the suluk, they comprehend the shadow of the Beautiful, or witness the origin of the Beauty, the Greatness hidden in the secret of the Beauty effaces them, and they go into rapture, for in every beauty there is a hidden glory, and in every glory there is a covered beauty. There is probably an allusion to this point in his saying when the guardian of the gnostics and of the saliks, Amirul Mu'minin (the commander of the faithful), may Allah bless him and all his offspring, says: "Glory to the One whose compassion is vast for His friends despite His severe revenge, and whose revenge is severe for His enemies despite His vast compassion," 
Thus, the Majesty, Greatness and Power of Beauty envelop them and they fall into a state of submission [khushu] before the Beauty of the Beloved. This state, at the beginning, causes agitation to the heart and engenders anxiety. Then, after submission [tamkin], it changes to a state of familiarity, and the agitation and anxiety, caused by the Greatness and Power, turn into familiarity and peace, and there happens a state of tranquillity, as was the state of the heart of khalilur-Rahman (Allah's Friend = Ibrahim) (AS).
Some other hearts are fearing and they are of the manifestations of Glory [jalal]. They are in continual understanding of the Greatness, Grandeur and Glory. Their submission is of fright, and the Subjugative and Majestic Names are manifested to their hearts, as was the state of the prophet Yahya (may Allah's peace be upon him and our Prophet and his progeny). So, submission is sometimes mixed with love, and sometimes with fear and fright, though in every love there is fear, and in every fear there is love.
The degrees of submission are according to the degrees of understanding the Greatness, Majesty and Beauty. Now as we, in our case, are deprived of the light of visions, we have but to indulge in acquiring submission by means of knowledge and faith [iman]. Allah, the Exalted, says: "Successful, indeed, are the believers who are submissive in their salats,"  Submission in the salat is regarded as a sign of faith [iman]. So, whoever is not submissive in his salat, will, according to Allah's saying, be excluded from the faithful [ahl-i iman]. Our salats which are not accompanied with submission are caused by a deficiency of faith or by lacking it. A belief [i'tiqad] and knowledge are other than faith, our knowledge of Allah, His Names and Attributes and of other divine Knowledge [ma'arij], is other than faith. Satan, according to the testimony of Allah, has information about the Beginning and the Resurrection, yet, he is a disbeliever. He said: "You have created me of fire, while You created him of dust,"  So, he believes in Allah and in His being the Creator; and he says: "Respite me until the day they are resurrected",  So, he believes in the Day of Resurrection, too. He knows about the Books, the Messengers and the angels. Nevertheless, Allah addresses him as a disbeliever, excluding him from the group of the believers [mu'minin].
Thus, the people of knowledge are distinct from those of faith. Not every man of knowledge is a man of faith. Therefore, after acquiring knowledge, one has to join the believers, and to convey the Greatness, Majesty, Brightness and Beauty of Allah, the Most Exalted and High, to his heart, so that it may become submissive, since mere knowledge does not result in submission. You can realize it in yourself: although you do believe in the Beginning and the Resurrection and in Allah's Majesty and Glory, your heart is not submissive. As to Allah's saying: "Has not the time yet come for those who believe that their hearts should be submissive when remembering Allah and what has come down of the truth,"  it may be that it is the formal faith the very belief in what the Prophet (SA) has brought - which is intended here, for the true faith is accompanied by a degree of submission; or the submission in the noble ayah may refer to a submission at its complete degree, as sometimes they apply the word, 'alim (erudite) to the one whose knowledge has reached the limit of faith. In the noble ayah: " ...verily only the erudite among Allah's servants fear Him, "  the reference may be to them. In the terms of the Book and the Sunnah, knowledge, faith and Islam refer to different degrees, the explanation of which is out of the scope of these papers.
Generally speaking, the salik on the way to the Hereafter -specially if with the mi'raj (ascending) step of the salat -will have to make his heart submissive by the light of knowledge and faith, so as to strengthen, as much as he can, this divine gift and the beneficent gleam, in his heart, trying to keep this state during the whole length of the salat. This state of consolidation and stability, though a bit difficult at the beginning for people like us, it becomes quite possible by practice and by exercising the heart.
My dear: Acquiring perfection and the provision for the Hereafter requires demand and seriousness, and the greater the demand, the more it deserves being serious about it. Certainly, with such a state of weakness, laxity and carelessness, one cannot ascend to the divine proximity and to be in a place neighbouring the Lord of Might. one has to manly set forth in order to reach what one wants. Since you do believe in the Hereafter, and find no way of comparability between that world and this -whether regarding their happiness and perfection, or their sufferings and calamities, as that world is eternal, with no death and perishing, where the happy live in comfort and dignity and in everlasting bliss, a comfort which has no like in this world, a divine glory and sovereignty, the like of which cannot be found in this life, and a bliss which never occurs in anybody's imagination, and similarly regarding the sufferings of that world, its pains, torments and evils can have no match in this world -you should know that the way to happiness runs through obeying Allah, the Lord of Might. None of the acts of worship and obedience can be on the same footing as that of the salat, which is a comprehensive [jami] divine mixture [ma'jun] undertaking the happiness of humanity. If it is accepted, all other acts (of worship) will be accepted. So, you are to exert utmost seriousness in obtaining it, never to feel tired in the quest, and to bear whatever hardship there may be, though there will be none. Actually if you continued it for a while and got cordially familiar with it, you would get, in this very world, so much pleasure out of your talk with Allah -a pleasure which cannot be compared with anyone of this world's pleasures. this will be quite obvious if we study the states of the people supplicating to Allah.
In general, to sum up our discussions in this chapter, we may say that, having comprehended the Greatness, the Beauty, and the Majesty of Allah, either by means of reasoning and proofs, or through the explanations of the prophets (AS), one must remind his heart of it, and then, by gradual remindings, cordial attention and continual remembering Allah's Greatness and Majesty, he has to bring about the state of submission in his heart, so as to attain to the required result. At any rate, the salik should not suffice himself with his present station, as any station which we may obtain does not worth a farthing in the market of the people of knowledge, nor it worths a grain of mustard in the bazar of "the people of heart". The salik must, in all situations, remember his own faults and deficiencies, so that he may find, through this, a way to his happiness. And praise be to Allah.
. Nahjul Balaghah, sermon 90.
. Surah al-Mu'minun: 1-2.
. Surah al-A'raf: 12.
 Ibid: 14.
. Surah al-Hadid: 16.
. Surah Fatir: 28.