Bg 12.5

kleśo ’dhika-taras teṣām
avyaktāsakta-cetasām
avyaktā hi gatir duḥkhaṁ
dehavadbhir avāpyate
Word for word: 
kleśaḥ — trouble; adhika-taraḥ — very much; teṣām — of them; avyakta — to the unmanifested; āsakta — attached; cetasām — of those whose minds; avyaktā — toward the unmanifested; hi — certainly; gatiḥ — progress; duḥkham — with trouble; deha-vadbhiḥ — by the embodied; avāpyate — is achieved.
Translation: 
For those whose minds are attached to the unmanifested, impersonal feature of the Supreme, advancement is very troublesome. To make progress in that discipline is always difficult for those who are embodied.
Purport: 

The group of transcendentalists who follow the path of the inconceivable, unmanifested, impersonal feature of the Supreme Lord are called jñāna-yogīs, and persons who are in full Kṛṣṇa consciousness, engaged in devotional service to the Lord, are called bhakti-yogīs. Now, here the difference between jñāna-yoga and bhakti-yoga is definitely expressed. The process of jñāna-yoga, although ultimately bringing one to the same goal, is very troublesome, whereas the path of bhakti-yoga, the process of being in direct service to the Supreme Personality of Godhead, is easier and is natural for the embodied soul. The individual soul is embodied since time immemorial. It is very difficult for him to simply theoretically understand that he is not the body. Therefore, the bhakti-yogī accepts the Deity of Kṛṣṇa as worshipable because there is some bodily conception fixed in the mind, which can thus be applied. Of course, worship of the Supreme Personality of Godhead in His form within the temple is not idol worship. There is evidence in the Vedic literature that worship may be saguṇa or nirguṇa – of the Supreme possessing or not possessing attributes. Worship of the Deity in the temple is saguṇa worship, for the Lord is represented by material qualities. But the form of the Lord, though represented by material qualities such as stone, wood or oil paint, is not actually material. That is the absolute nature of the Supreme Lord.

A crude example may be given here. We may find some mailboxes on the street, and if we post our letters in those boxes, they will naturally go to their destination without difficulty. But any old box, or an imitation which we may find somewhere but which is not authorized by the post office, will not do the work. Similarly, God has an authorized representation in the Deity form, which is called arcā-vigraha. This arcā-vigraha is an incarnation of the Supreme Lord. God will accept service through that form. The Lord is omnipotent, all-powerful; therefore, by His incarnation as arcā-vigraha He can accept the services of the devotee, just to make it convenient for the man in conditioned life.

So for a devotee there is no difficulty in approaching the Supreme immediately and directly, but for those who are following the impersonal way to spiritual realization the path is difficult. They have to understand the unmanifested representation of the Supreme through such Vedic literatures as the Upaniṣads, and they have to learn the language, understand the nonperceptual feelings, and realize all these processes. This is not very easy for a common man. A person in Kṛṣṇa consciousness, engaged in devotional service, simply by the guidance of the bona fide spiritual master, simply by offering regulative obeisances unto the Deity, simply by hearing the glories of the Lord, and simply by eating the remnants of foodstuffs offered to the Lord, realizes the Supreme Personality of Godhead very easily. There is no doubt that the impersonalists are unnecessarily taking a troublesome path with the risk of not realizing the Absolute Truth at the ultimate end. But the personalist, without any risk, trouble or difficulty, approaches the Supreme Personality directly. A similar passage appears in Śrīmad-Bhāgavatam. It is stated there that if one ultimately has to surrender unto the Supreme Personality of Godhead (this surrendering process is called bhakti), but instead takes the trouble to understand what is Brahman and what is not Brahman and spends his whole life in that way, the result is simply troublesome. Therefore it is advised here that one should not take up this troublesome path of self-realization, because there is uncertainty in the ultimate result.

A living entity is eternally an individual soul, and if he wants to merge into the spiritual whole, he may accomplish the realization of the eternal and knowledgeable aspects of his original nature, but the blissful portion is not realized. By the grace of some devotee, such a transcendentalist, highly learned in the process of jñāna-yoga, may come to the point of bhakti-yoga, or devotional service. At that time, long practice in impersonalism also becomes a source of trouble, because he cannot give up the idea. Therefore an embodied soul is always in difficulty with the unmanifest, both at the time of practice and at the time of realization. Every living soul is partially independent, and one should know for certain that this unmanifested realization is against the nature of his spiritual, blissful self. One should not take up this process. For every individual living entity the process of Kṛṣṇa consciousness, which entails full engagement in devotional service, is the best way. If one wants to ignore this devotional service, there is the danger of turning to atheism. Thus the process of centering attention on the unmanifested, the inconceivable, which is beyond the approach of the senses, as already expressed in this verse, should never be encouraged at any time, especially in this age. It is not advised by Lord Kṛṣṇa.