In John 3:15 John makes the comment that the purpose of Jesus' being lifted up is so that "those who believe might have life in him."
A couple of things come to mind. First, the verb "believe" is used with no object. Taken on its own one might be tempted to think that one must simply be "a person of faith" in order to qualify for eternal life. Surely that is the theological direction that some denominations, most notably the RCC, are drifting towards. This would then promote the idea that being a sincerely religious person is enough. Because you are person who at least believes (apparently this is a meritorious virtue) God will grant you a pass for believing the wrong things.
But this is a good example of why we need to keep verses in context. Doing so helps us to see that we are not free to simply assume that there is no defined content to "believing". For in John 3:12 we see that Jesus includes, at least, trust and faith in himself and his teaching.
That, incidentally, is why I don't think we can say that Nicodemus is, at this point, a believer in Christ. He can say that Jesus is from God, but doesn't understand how much of an understatement that is. He s in fact incredulous about some of what Jesus teaches, and disbelieves some as well. The examples are not deep or minute theological details either, they are basics.
We really do need to be on guard against the notion that sincerity covers a multitude of sins. We need to believe, yes, but we have to believe in something, someone, specific in order to be saved.
The RCC is, IMO, leading people down the garden path in this aspect.
The RCC has this notion of a "baptism of desire" which posits that if a person is sincere they would want to be baptized (they believe baptism regenerates the person, is the instrument of the New Birth Jesus speaks of) and that's what counts. That's rather patronizing. Sincere practitioners a of other beliefs are not Christians, but they will be some day, regardless of what they believe now.
That, as far as I am concerned, is hogwash. There is nothing virtuous about being a person of faith as such in terms of salvation. The Essenes were sincere practitioners of their faith, and the Sadducees were as well. The sadducees were still taken to task by Jesus.
There is no substitute for a real, personal faith. Not in God's eyes.