Text 19

iti kshetram tatha jnanam
jneyam coktam samasatah
mad-bhakta etad vijnaya
mad-bhavayopapadyate

Translation

Thus the field of activities [the body], knowledge and the knowable have been summarily described by Me. Only My devotees can understand this thoroughly and thus attain to My nature. 

Commentary by Srila Prabhupada 

The Lord has described in summary the body, knowledge and the knowable. This knowledge is of three things: the knower, the knowable and the process of knowing. Combined, these are called vijnana, or the science of knowledge. Perfect knowledge can be understood by the unalloyed devotees of the Lord directly. Others are unable to understand. The monists say that at the ultimate stage these three items become one, but the devotees do not accept this. Knowledge and development of knowledge mean understanding oneself in Krishna consciousness. We are being led by material consciousness, but as soon as we transfer all consciousness to Krishna’s activities and realize that Krishna is everything, then we attain real knowledge. In other words, knowledge is nothing but the preliminary stage of understanding devotional service perfectly. In the Fifteenth Chapter this will be very clearly explained. 

Now, to summarize, one may understand that verses 6 and 7, beginning from maha-bhutani and continuing through cetana dhrtih, analyze the material elements and certain manifestations of the symptoms of life. These combine to form the body, or the field of activities. And verses 8 through 12, from amanitvam through tattva-jnanartha-darshanam, describe the process of knowledge for understanding both types of knower of the field of activities, namely the soul and the Supersoul. Then verses 13 through 18, beginning from anadi mat-param and continuing through hridi sarvasya visthitam, describe the soul and the Supreme Lord, or the Supersoul. 

Thus three items have been described: the field of activity (the body), the process of understanding, and both the soul and the Supersoul. It is especially described here that only the unalloyed devotees of the Lord can understand these three items clearly. So for these devotees Bhagavad-gita is fully useful; it is they who can attain the supreme goal, the nature of the Supreme Lord, Krishna. In other words, only devotees, and not others, can understand Bhagavad-gita and derive the desired result.

Commentary by Sri Visvanatha Cakravarti Thakur

In this verse the Lord summarizes the topic about the field and other items previously described along with the result for those who practice. The field consists of the items starting with the five gross elements and ending with dhrti, perseverance in verses 5 and 6. Knowledge was described starting with pridelessness (amanitvam) and ending with reviewing the goal of liberation (tattvajnartha darsanam) in verses 7 through 11. The object of knowledge to be attained by the process of knowledge was described in verses 12 through 18 starting with the word anadi and ending with the word dhistam. The one truth known as brahman, bhagavan and paramatma has thus been briefly described.

The jnani with some devotion (mad bhaktah), knowing this, merges with me (mad bhavaya). An alternate meaning of the sentence is "My ekanta bhakta, understanding that I, his master, have such powers (etad vijnaya), becomes endowed with prema (mad bhavaya) for me."