The Jesus of the time of Jesus faced one challenge because they already had Moses. While they were keeping Moses, there came the time to forego the annual justification of the Day of atonement and believe in Jesus as the new means of atonement.
It is not the case for the world to just believe in Jesus and that's it because salvation is a free gift of God and not from works.
'Once saved always saved' is not biblical but another lie of Satan because Philippians 2:12 calls on us to work out our salvation with fear and trembling and Ezekiel 33:13 tops it all in declaring if we commit one iniquity, God will forget all our goodness and we will die.
We cannot work out something that is already set and settled.
Christ did not get victorious over sin so that we do not have to get our own victory. If that were so, He would not have called on us to repent (Luke 5:32). Jesus is waiting on us to get victory over sin, according to Revelation 3:20-21. That passage does not say that we will seat with Him on His throne because He overcame for us.
We must win our individual battle against sin, even if it comes to death. As such, lip belief in Jesus does not save because many who claim to be saved denied that Jesus is I AM or YAHWEH God, according to the declaration of Jesus in John 8:24.
The victory of Jesus over sin was not for us so that we do not have to seek our own victory over our sins.
Philippians 2:12 YLT
(12) So that, my beloved, as ye always obey, not as in my presence only, but now much more in my absence, with fear and trembling your own salvation work out.
Ezekiel 33:13 ASV
(13) When I say to the righteous, that he shall surely live; if he trust to his righteousness, and commit iniquity, none of his righteous deeds shall be remembered; but in his iniquity that he hath committed, therein shall he die.