The Sikhs in Amritsar formed the Sanatan Singh Sabha as the original Singh Sabha in 1873, "It was essentially conservative and Sanatan ('eternal', almost synonymous with Hinduism). The Sanatan Sikhs (Udasis, Nirmalas and the Namdharis) were for the first time challenged and eventually marginalised.".
Origin
The Sanatan Sikhs refer to the Classical Sikhism as Sikhs to be a wider denomination of Sanatana Dharma by the individual who practices karma and Bhakti of the Almighty in any way for the achievement of Moksha, or spiritual liberation. Sikh groups have criticized the Sanatan Singh Sabha as having more in common with Hinduism than Sikhism.
References
Sanatan Singh Sabha Wikipedia(Text) CC BY-SA