While some people believed in Jesus, others rejected him. Many Pharisees and religious leaders reject Jesus, but there are instances where teachers and rulers like Nicodemus are more open to Jesus. In fact, many rulers believed in Jesus (12:42).
However, the rulers did not confess Jesus because of the Pharisees (12:42). The Pharisees kicked out people who favored Jesus (remember the healed man in John 9?). The rulers cared about public perception more than God's praise (12:43), and getting kicked out of a synagogue (a common meet place) does not look good.
Because the rulers believed in Jesus, they are guaranteed everlasting life (as written in John 3:16 and 5:24). However, their fear of their own reputation upon encountering the Pharisees is the limiting factor. Because of their fear, the rulers decided not to confess Jesus openly. Fear keeps them from sharing Jesus, and sharing Jesus creates impact (remember the Samaritan woman in John 4?). It's worth noting that the fear is related to a focus more on praise of men than praise of God, so focusing on God more than public perception can counter said fear.
Today, believers can sometimes have fear regarding public perception that keeps them from proclaiming Jesus. However, focusing on God more than public perception can help us overcome said fear.