*How to know and unite with the Bhagavan?*

O Arjun, by unalloyed devotion alone can I be known as I am, standing before you. Thereby, on receiving My divine vision, O scorcher of foes, one can enter into union with Me.

~ Gita 11.54

By single-minded devotion only the Lord could be known, seen, and attained.  

The mind should not be attached to anything in the objective world; it should be centred in the Lord as the sole refuge, as the most beloved and as the highest goal.

Human mind is generally possessed by worldly things.

When the mind is not distracted by anything material, it acquires divine power and thus attains the Lord.

Three stages of God-realisation are mentioned here: (1) to be known, (2) to be seen, (3) to be entered into. 

In the beginning, a person acquires knowledge of the nature of the Lord. Even this is difficult, because it requires deep study and correct understanding of the Sastras. 

At this stage, the Lord, though his nature is understood, is quite far away. There is a distinction between human and God. This is Dwaita (duality).

Then in the next stage, the devotee is able to see the Lord face to face, have a clear vision of God in whatever form he contemplates Him. Now there is no doubt at all about His existence, because he has directly seen the Deity. 

The second step is higher than the first.

After long and arduous practice the devotee becomes united with the Lord.

Like salt in water, like rain drops in the ocean, like water in milk, the devotee is merged in the Lord. 

Then there are no two entities, the devotee and the Lord. The Lord alone remains. This is the highest stage. This is perfect Advaita. 

The devotee should not stop anywhere till this highest goal is reached.