The bible isnt "either or" it is "and, and with".
Just Believing in Jesus is not enough or demons would be saved.
James 2:19 You believe that God is one. You do well; the demons also believe, and shudder.
Luke 4:41 Demons also were coming out of many, shouting, "You are the Son of God!" But rebuking them, He would not allow them to speak, because they knew Him to be the Christ.
Jesus also says we must do the will of his Father (Matthew 7:21) He says you must eat his flesh and drink his blood (John 6:58), He says you "must" be born again of "WATER" and Spirit (John 3:5, he says you must be charitable(Mark 9:41), He says you must forgive others to be forgiven yourself(Matthew 6:12), he says to let your Good Works shine before God (Matthew 5:16).....
Beliefism is not supported by scripture.
Here is something else Jesus says:
John 10:1 "Truly, truly, I say to you, he who does not enter by the door into the fold of the sheep, but climbs up some other way, he is a thief and a robber.
Jesus tells us we must believe in him, repent, be born again of water and spirit, eat his flesh and drink his blood, obey the commandments.