Krishna sounds pretty far-fetched. Horus sounds more plausible. There was a lot of syncretism in the Hellenistic period and that carried into Gnostic Christianity, there's clear elements as well in Essene Judaism and even in the Johannine school. Look at how mainstream Judaism soaked up Purim and the concepts of hell, paradise (a Persian word), Satan from the Persians. But things are more complex than just borrowing a notion lock-stock-and-barrel from one religion to another. The Persian practice of consecrating the ground for Ahura Mazda fed into the preexistent Hebrew practice of pining the executed on a tree on the day of execution -- and it is fitting that when the Romans adopted crucifixion from the Phonecians after the Punic Wars (which ultimately is a syncretism of Persian crucifixion and Roman patibulum torture), it was widely practiced in Judaea for it matched already-existing ritual practices. If a community is committed to conservative ritual practices (circumcision, dietary laws), as the nascent Christian church was, and if it saw itself opposed to Hellenizing tendencies, as early orthodox Christianity was (cf. the conflict with Marcion), it is hard to believe that the whole central concept of Jesus as Savior was directly adopted from some mystery cult dedicated to Horus, a solar deity, or what not. If there was a pagan antecedent to Jesus' saviorhood, it would've already been smuggled into the Messianic current of Judaism which Christianity later drew from. For a Hellenistic parallel to the figure of Jesus, see the case of Apollonius of Tyana.
Look at how such blatantly Persian concepts of paradise and hell were Judaized into such underworld regions as the Bosom of Abraham and Gehenna (see the discussion in Josephus on the geography of Hades).
That does not deny the reinforcing effect of mystery cults on the
themes of Christianity. In the religious landscape of the Pax Romana, Christianity took its place among the various other mystery cults, and while some branches of Christianity more overtly adopted Hellenistic influences, more mainstream orthodox Christians saw the pagan parallels as imperfect, demonic anticipations of the truth about Christ. Justin Martyr alluded to myths about the sons of Zeus and said "when we say also that the Word, who is the First-begotten of God, was born for us without sexual union, Jesus Christ our teacher, and that He was crucified and died and rose again and ascended into heaven, we propound nothing new beyond what you believe concerning those whom you call sons of Zeus."
If you want to posit such a role of pagan soteriological fiction in the development of Christianity, it seems to me that the concepts were already Judaized to the extent that Jesus either historically or post facto fit into the expected role, and his saviorhood was deemed within a mainstream Jewish framework, except by groups that leaned more overtly to contemporary Hellenistic thought (and which were rejected later by the "apostolic" churches). But even mainstream Christianity continued to absorb Hellenistic currents, such as the Mary cult from Isis cults and Easter and Christmas from the solar god (dying-rising) themes of Mithraism.
Leolaia