Probably the answer you are looking for is the Jews or Israelites, but that is not precisely accurate, because “Old Testament” is really a Christian phrase / concept that, while overlapping with much of the Tanakh (the Jewish version of the Bible)... read more
My Dear Friend, the followers of old testament now you can say are Muslims instead of Christians because what ever is written in the old testament is reflecting in Quran. The Adam sin, The Abraham story etc., the son of Abraham who is Ismail’s followers are Muslims, The Isaac followers are Christians as per the information i got. read more
An index in the Complete Jewish Bible catalogs 695 separate quotations of Old Testament passages in the New Testament (David Stern, 1998, pp. 1610-1615). In dozens of additional places the Old Testament is referred to (as in cases where an Old Testament figure is mentioned), but no specific scripture is quoted. read more