He whom God shall make manifest is a messianic figure predicted by the Báb within his book the Bayán that would come after him and lead the Bábís. A prediction widely recognized as being fulfilled by Bahá'u'lláh.

Dayyán's claim

Some time in the 1850s, Mirza Asadullah-i-Khuy often named Dayyán was one of the first to claim this position however he was assassinated before he could attract many more than a few followers. Bahá'u'lláh indicated that it was Subh-i-Azal, a leader of the Bábís, who ordered the killing by decree.

Nabil's claim

Nabíl-i-A'zam of Zarand, the author of the book 'The Dawn Breakers', made a claim around the same time as Dayyán or perhaps shortly afterward, but withdrew his claim and became a staunch supporter of Bahá'u'lláh.

Bahá'u'lláh's claim

Many Bábís believed in Bahá'u'lláh's claim, first mentioned in 1863, but publicly announced around 1868. Those who followed him became known as Bahá'ís. This claim was by far the most successful.

Subh-i-Azal's claim

Shortly after Bahá'u'lláh's claim first started to attract attention, Subh-i-Azal is said to have made a claim to the title by Bahá'í historians. It is unclear whether this is contested by Subh-i-Azal's followers as little non-Bahá'í material covers the topic. If the claim was made, it was retracted shortly later.

Notes

1. In his letter, the Epistle to the Son of the Wolf, Bahá'u'lláh directly fingered Subh-i-Azal as the origin of the order: "suffered martyrdom through the decree pronounced by Mirza Yahya [Subh-i-Azal".
2. This is covered by Shoghi Effendi in God Passes By: "When Bahá’u’lláh formally declared His Mission to him in Adrianople, Mirza Yahya ^* responded by going to the length of putting forward his own claim to be the recipient of an independent revelation". The Universal House of Justice reiterates this in at least one letter to a Bahá'í, although most likely uses the former as its source.

References

Bahá'í history