Important verse of Srimad bhagvad gita -18

I am never manifest to the foolish and unintelligent. For them I am covered by My eternal creative potency [yoga-maya]; and so the deluded world knows Me not, who am unborn and infallible....7:25





Now Lord Krishna explains the cause of their ignorance about Himself. It is because He only manifests Himself in and through the consciousness of His devotees exclusively and not to others

Why are not all beings able to acquire knowledge about the Supreme Lord ?  The Supreme Lord Krishna whose potencies are beyond the realm of the mind and the senses and who is the sole objective of meditation by the enlightened yogis. For others He is imperceptibly veiled by His mysterious and intrepid, internal energy known as yogamaya. Yoga means uniting with, maya is illusory impressions superimposed upon the consciousness, by this combination a mysterious power arises which is transcendant to material nature and has the potency at any time to make the impossible, possible or the possible, impossible or even have them both manifesting simultaneously. The ignorant are covered by this maya and hence unaware of Lord Krishna's supreme position. Thus being oblivious to His powers, glories, qualities and attributes the ignorant are unable to have communion with Him and receive His grace.
Why are not all beings able to acquire knowledge about Him?
Lord Krishna speaks naham prakasah sarvasya meaning He is not revealed to everyone. He is samavrtah or concealed. He allows the ignorant who are bereft of faith be oblivious of His divine glory and His purely spiritual form which is endowed with qualities and attributes that are completely transcendental to prakriti or the material substatum pervading all worldly objects in the physical existence. His supernatural powers and lilas or phenomenal pastimes are not known, heard or witnessed by all. Only those who are exclusively devoted to Lord  are aware of His power and majesty. As the Supreme Lord Himself spoke to Narada Muni that: One by one, then two by two, then in a group of three, great sages desired to see the form of the Supreme Lord ; but they were unable to, nor will they ever be able too until they have developed exclusive devotion for Him. Only by bhakti or exclusive loving devotion can the Supreme Lord  be known. Hence the ignorant, all who are not Lord 's fail to recognise Him as the Supreme Being. For He is not born as an embodied soul forced to accept a physical body like all embodied beings by the dictates of karma or reactions from previous actions. Lord  manifests Himself  by His own sweet will to perform His divine lilas or phenomenal pastimes for sport; but the people of the world merely view Him as another human being only blest to have amazing and extraordinary qualities and characteristics.


vedaham samatitani 
vartamanani carjuna 
bhavisyani ca bhutani 
mam tu veda na kascana.......7:26


O Arjuna, as the Supreme Personality of Godhead, (I) know everything that has happened in the past, all that is happening in the present, and all things that are yet to come. I also know all living entities; but Me no one knows...7:26

Lord Krishna emphasises by the words mam tu veda na kascam which means that no one is able to know Him and because He is the controller of all, anywhere that He is not veiled is only by His express directive for His veiling is not for His devotees but for those who are in the clutches of maya or illusory impressions superimposed upon the mind, so that they will not be able to know Him. This He is indicating by the word vedaham meaning He knows. That He knows by His divine potency and unveiled knowledge. Although His external energy known as maya veils the mind in varying degrees of all created beings; it should clearly understood that that all living beings movable or immovable, belonging to the past, existing in the present or manifesting in the future are under His complete control and this is an example of how He is omniscient. 


NOte:  His veiling is not for His devotees but for those who are in the clutches of maya or illusory impressions superimposed upon the mind, so that they will not be able to know Him.

or it can be said परन्तु मुझको कोई भी 1) अनन्य  भक्ति  2) श्रद्धा  से जाना जासकता है . 
Hence be in bhakthi by bhajan and keep faith and walk on the path to know higher consciousness from the present level of lower consciusouness by various methods of yoga, either by karma yoga ( by doing action), ananya bhakti yoga(  getting united through form of perpetual bhakthi), gyana yoga( through the knowledge of reality) yoga means uniting .