In plain words
In this way, the duty devotees have to carry out is precisely this: to practise the actions fit to make their resolves pure. From then on Mārkaṇḍeya has lived on, never to die. He went on giving joy to his parents. God's will never changes. That will goes on being done, without fail. But through a devotee's selfless, spotless prayers, some changes come about. In the devotee there is a power that can change even God's will. Bhakti does not mean mere empty words.
The liṅga is the emblem of the nature of Īśvara
In the seventh century Ādi Śaṅkara was born in the land of Kerala. He was born, and with the aim of teaching God's will to every human being in the world, he went on proclaiming the meaning of all the śāstras. But Śaṅkarācārya, great as he was, lived only a short span of thirty-two years. Later, in the eleventh century, Śrī Rāmānujācārya was born. ‘In the world Bhakti and prapatti are dwindling away; in man, what is called Bhakti is becoming a void. The very thought of God is vanishing. That must be set on its feet again’: and so he came to teach the special worth and the importance of remembering the name of God. Then in the thirteenth century came Śrī Madhvācārya. He went on teaching that the jīva and God are two, separate from each other. But though the jīva and God are separate, what is in both is one and the same principle of the Ātma. The principle of the Ātma in every being is one and the same. The nature of Īśvara in all is one and the same. In the world, what is called the nature of Īśvara is not something made up. It is something that comes naturally, of itself. Rāma came bearing a body. Kṛṣṇa too came bearing a body. This bodily principle is not the nature of Īśvara. One's own principle of Satya is itself the nature of Īśvara. That is, the oneness of the three, creation, sustenance and dissolution, lies in the very nature of Īśvara. On just this basis the learned pictured the shape of the liṅga as the emblem of the nature of Īśvara.