sentence correction

Martin luther king was one of the leaders (1)/who (2)/has followed Mahatama Gandhi (3)/no error

in this sentence have will come in place of has according to leaders which is plural.
my query is:
we are talking of martin luther who is singular. therefore why we dont use the helping verb according to first subject that is martin?

Secondly, one of is followed by plural noun and singular helping verb, therfore it should be who had or has followed.

pls suggest your answers with reasons.
Thank you.