Actual devotional service continues even after liberation. When the devotee goes to the spiritual planet in the kingdom of God, he is also engaged there in serving the Supreme Lord. He does not try to become one with the Supreme Lord
Bhagavad-gita As It Is
BG Chapters 7 - 12
The process of devotional service is a very happy one (su-sukham). Why? Devotional service consists of śravaṇaṁ kīrtanaṁ viṣṇoḥ (SB 7.5.23), so one can simply hear the chanting of the glories of the Lord or can attend philosophical lectures on transcendental knowledge given by authorized ācāryas. Simply by sitting, one can learn; then one can eat the remnants of the food offered to God, nice palatable dishes. In every state devotional service is joyful. One can execute devotional service even in the most poverty-stricken condition. The Lord says, patraṁ puṣpaṁ phalaṁ toyam: He is ready to accept from the devotee any kind of offering, never mind what. Even a leaf, a flower, a bit of fruit, or a little water, which are all available in every part of the world, can be offered by any person, regardless of social position, and will be accepted if offered with love. There are many instances in history. Simply by tasting the tulasī leaves offered to the lotus feet of the Lord, great sages like Sanat-kumāra became great devotees. Therefore the devotional process is very nice, and it can be executed in a happy mood. God accepts only the love with which things are offered to Him.
It is said here that this devotional service is eternally existing. It is not as the Māyāvādī philosophers claim. Although they sometimes take to so-called devotional service, their idea is that as long as they are not liberated they will continue their devotional service, but at the end, when they become liberated, they will "become one with God." Such temporary time-serving devotional service is not accepted as pure devotional service. Actual devotional service continues even after liberation. When the devotee goes to the spiritual planet in the kingdom of God, he is also engaged there in serving the Supreme Lord. He does not try to become one with the Supreme Lord.
As will be seen in Bhagavad-gītā, actual devotional service begins after liberation. After one is liberated, when one is situated in the Brahman position (brahma-bhūta (BG 18.54)), one's devotional service begins (samaḥ sarveṣu bhūteṣu mad-bhaktiṁ labhate parām). No one can understand the Supreme Personality of Godhead by executing karma-yoga, jñāna-yoga, aṣṭāṅga-yoga or any other yoga independently. By these yogic methods one may make a little progress toward bhakti-yoga, but without coming to the stage of devotional service one cannot understand what is the Personality of Godhead. In the Śrīmad-Bhāgavatam it is also confirmed that when one becomes purified by executing the process of devotional service, especially by hearing Śrīmad-Bhāgavatam or Bhagavad-gītā from realized souls, then he can understand the science of Kṛṣṇa, or the science of God. Evaṁ prasanna-manaso bhagavad-bhakti yogataḥ. When one's heart is cleared of all nonsense, then one can understand what God is. Thus the process of devotional service, of Kṛṣṇa consciousness, is the king of all education and the king of all confidential knowledge. It is the purest form of religion, and it can be executed joyfully without difficulty. Therefore one should adopt it.