But, O Partha, the great-souled ones, possessing the divine Nature, knowing Me as immutable and as the source of beings, worship Me with single-minded devotion. (Bhagavad Gita Chapter 9 verse 13).
The great-souled ones, the mahatmas, those who are of sattvic, purified mind, My faithful bhaktas, who walk in the path of moksha, regarding Me as their own innermost Self; those who possess the divine Nature, the nature of the devas; who have control over body, speech, mind and senses; who are kind, charitable and full of faith — they know Me as unchangeable, as the origin of all beings, of all living creatures and also as the origin of the elements of matter. Knowing Me thus, they worship Me with single-minded devotion.
Other quotes on the topic of single-minded devotion
Through single-minded devotion alone, I can be seen in this (four-armed) form, can be known in essence, and also can be reached, Arjuna. The one who does all works for Me, and to whom I am the supreme goal, who is my devotee, who has no attachment, and is free from enmity towards any being attains Me, O Arjuna. (Bhagavad Gita -54 – 55)
If you have earnest longing, you can attain Bhagavan by any path. But you must be steadfast. There is another name for single-minded devotion – chaste devotion. Think of a tree with a single trunk. It grows straight up. Other devotion is like a tree with five branches. The gopis of Vrindavan had such single-minded devotion that they didn’t want to see any person but the cowherd Krishna of Vrindavan wearing his peacock feather crown and yellow loin-cloth. When they saw Krishna in Mathura in royal dress with a turban on his head, they drew down their veils and said – ‘Who is he? Should we become unchaste by talking to him? (Sri Ramakrishna Paramahamsa).
Hanuman had single-minded devotion. When he came to Dwaraka in the Dwapar Yuga, Bhagavan Sri Krishna said to Rukmini, - Hanuman won’t be happy unless he sees me in the form of Sri Rama. So Bhagavan assumed the form of Rama to satisfy the single-minded devotion of Hanuman.
You don’t believe in God with form. There is no harm in that. It’s enough to have single-minded devotion to the formless God. Even so, you should cultivate the longing that the believers in God with form have.
When a person becomes truly motivated to know, to see and to merge with the highest consciousness that has been called God, the prerequisite and the process is single-minded devotion.
The cosmic form of bhagavan can be seen and understood only through single-minded devotion.