Just so we are clear:
Imagine all you have to do is believe in Jesus as your saviour and voila-eternal life. Like the magic elixor at a circus. Believe and you have an instant fix.
Believe means the following according to Strong's:
- to think to be true, to be persuaded of, to credit, place confidence in
- of the thing believed
- to credit, have confidence
- in a moral or religious reference
- used in the NT of the conviction and trust to which a man is impelled by a certain inner and higher prerogative and law of soul
- to trust in Jesus or God as able to aid either in obtaining or in doing something: saving faith 1bc) mere acknowledgment of some fact or event: intellectual faith
- to entrust a thing to one, i.e. his fidelity
- to be entrusted with a thing
"Just believe" is quite an understatement.
True, some evangelists do seem to promote the idea that "all you have to do is confess Jesus is your Lord and Savior" (say a magic prayer) to be saved but theologians agree that confessing you "believe" is only the beginning of the story.
Christians believe you must completely trust in Jesus and commit to a life subservient to Him. See, we are first "justified" by our confession but our "santification" is the hard part. A true act of justification will follow with a work of Jesus in our lives where we are "impelled by a certain inner and higher prerogative and law of soul."