Is it necessary to learn all the scholarly spiritual knowledge to attain bhakti and karma yoga or is just the essence of it enough?

[An online spiritual discussion was conducted on October 24, 2020, in which several devotees participated. Some of the questions of devotees answered by Swāmi are given below.]

The above question was asked by Śrī Durgaprasad. Śrī Phani added to the question by asking: Then how were devotees like Kaṇṇappā, the Gopikās and Śabarī who were illiterates, able to reach the climax of devotion?

Swāmi replied: O Learned and Devoted Servants of God! The souls born as devotees and servants of God like Kaṇṇappā, the Gopikās, Śabarī etc., had already attained jñāna yoga or spiritual knowledge over the past several births (Labhate ca tato bhūyaḥ…—Gītā). Jñāna yoga should always be attained in an elaborate manner, so that no doubt remains. Learning just the essence is very good, but it does not clear all the doubts.