Belief in Jesus IS absolutely necessary for salvation.
However...
Hebrews 10:26
"Dear friends, if we deliberately continue sinning after we have received knowledge of the truth, there is no longer any sacrifice that will cover these sins".
I have asked you this several times now:
Are you saying that Christians do not need to keep God's commandments?
I believe God calls us to remain faithful. In my understanding human belief or human faith is involved with spiritual persuasion or spiritual faith.
It isn't keeping the law that saves. It is by us putting our faith in the only hope for salvation. A free gift to all who will hear and believe.
In my view, one can fall away by choice. How does this happen? By suppressing the Spirit and the commandments or will of God. Sanctification isn't for salvation. It is God training us to be Holy. There are things in this world that can choke us, tempt us, and hurt our relationship with God. By following God's will, we keep a healthy relationship. By being disobedient or even lazy we can create a poor relationship. A poor relationship invites infidelity or God would even call His people a whore. And would constantly time and time again, call for His people to return.
But ultimately the Apostles and Jesus simply called for belief. If one was to turn or apostate then what hope do they have if they return and remain in unbelief?
People debate this as people have for centuries, people will disagree with me and that's okay, we all are at different points in our faith journey and God will lead those obediently searching for truth in the right direction, in His time. To me the most important part of salvation is the beginning. If you believe that Jesus Christ is your Savior and Lord then everything else can be ironed out in time.