Lord Krsna has explained the preposition of one who contemplates upon nirguna-brahman as oneself. It is very difficult before one can fix one's mind on Īśvara in the form of entire jagad. Thus Lord Krsna said...
kleśo' dhikatarastesām avyaktāsaktacetasām | avyaktā hi gatirduhkham dehavadbhiravāpyate ||12.5|| Greater is the affliction for those whose minds are committed to what cannot be objectified, for, an end which cannot be objectified is reached with difficulty by those who are identified with the body.
kleśah adhikatarah tesām, there is greater affliction for them who contemplate upon aksara-brahman. Greater in comparison with affliction of the karma-yogī who meditate upon saguna-brahman. As a karma-yogī, one cannot always do the things one likes. Every action is based on dharma which sometimes is not convenient and not pleasing. In the case of meditation, it is not easy to visualise everything is Īśvara alone. Even keeping the mind focused on one form is difficult. Even though there are great difficulties for the karma-yogī who is totally committed to Īśvara, the difficulties of those committed to the pursuit of aksara-brahman are even greater, because of deha-abhimānam - identification with the body. Everybody has a body, and identification need to be there for doing any transaction. But the identification here means taking the attributes of body as my attributes, I am as good as my body. Nothing is more difficult than giving up identification with one's own body-mind-sense-complex. There are people who don't care about their own well being, but only worry about their family. That very attachment is through the body only, therefore negating the identification with body is important. But it can't just happen after knowing from śāstra that this body-mind-senses are not me. Even if one has come to understand this fact, there seems to be a parallel line of thinking which has nothing to do with knowledge. Because certain pattern of thinking just overwhelm this knowledge. Therefore removing the habitual error in the daily life is needed, where the life of karma-yoga provides circumstances for it. Going through experiences with an attitude of karma-yoga, giving up the sense of ownership, the identification with the body will reduce in course of time. Therefore it is very clear why Lord Krsna said that karma-yogī is the most exalted. Without giving up the old identity that I am as good as this body-mind-sense-complex, the new identity that I am limitless Brahman will not take place. Therefore, to make easy for one to see oneself as aksara-brahman, one should have a good training in karma-yoga. This is what Lord Krsna explains in next three verses.
ye tu sarvāni karmāni mayi sannyasya matparāh |
ananyenaiva yogena mām dhyāyanta upāsate ||12.6|| However, those who worship Me, keeping me as the ultimate end, giving up all actions unto Me, meditating upon Me with a commitment in which there is indeed no other...
tesām aham samuddhartā mrtyusamsārasāgarāt |
bhavāmi na cirāt pārtha mayyāveśitacetasām ||12.7|| ...for them, whose minds are absorbed in Me, Pārtha (Arjuna)! before long I become the liberator from the ocean of samsāra that is fraught with death.
tu - however when one finds that understanding of aksara-brahman as oneself is difficult, one should prepare oneself better in life by dedicating / offering one's actions to Īśvara in keeping with one's own duty and accepting the result of actions without feeling unfair. Bringing the understanding in one's action is important in this stage. Take the law of gravity as example, as we do not see the law itself until we see a falling object, in the same way we do not see dharma until dharmic-action is done. Therefore until the understanding of oneness has abided in oneself, karma-yoga provides an infrastructure to form a mature mind to see this oneness. This is the life of karma-yoga attitude, keeping Īśvara as the ultimate end and meditate upon Īśvara where there is no others.
As the result of karma-yoga, Īśvara become the liberator from the ocean of samsāra - samuddhartā mrtyusamsārasāgarāt. This life of becoming is compared to the ocean which is fraught with sorrow and difficult to cross. It is not easy to get out of it, therefore we need someone to liberate. But Īśvara can't liberate us by giving us the freedom from this life of becoming, because liberation from samsāra is only by knowing the true nature of oneself which is limitless, and this nature can't be given, it is always there. The only problem is we do not know, and the only solution is to make us know. Īśvara can give his grace to us in the form of situation necessary for jñānam, but knowledge only takes place when mind is well prepared. Whatever given by Īśvara cannot be the cause for moksa, because moksa is not created, it is only need to be known. Therefore we need to work in any area which will bring maturity and composure of the mind for knowledge to take place.