"Repent and believe in the gospel." This word believe is not merely an agknowledgment but a placing ones trust or confidence in something. It is the same for the word "faith". The idea is placing your trust and confidence in the finished work of Jesus on the cross which is the gospel. Jesus knew that his audience was placing their trust in other things to get them into heaven and to bring them hope and fulfillment. He also knew that He was the only way, truth and life. He knew that if any of these people were left in the state they were in of trusting in their own righteousness, they would remain also under God's wrath. Since God's heart is that none would perish, Jesus' heart was the same. This supernatural love drove Him to say things to people like "Repent and believe in the gospel". He did not tell people that God is at the top of a mountain and which ever side of the mountain you go up doesn't matter because they all lead to the same place. He told them to zero in on the Him. He told them to place their confidence and trust in His righteousness.
These three things are the key to true salvation. Repentance and belief in the gospel. If you leave one out, salvation won't happen. Jesus didn't say to repent only because what good would it do to turn from falsehood unless you knew what truth to turn towards. Jesus didn't leave out repentance from sin because sin is the very thing that seperates us from God. Neither did Jesus just explain the gospel to people without commanding to repent and believe lest anyone think that simply agknowledging His work on the cross would save them. For sceptics, Jesus' words are your clear warning and offer of a way out. For us christians, this is our model for sharing our faith.
Listen to Jesus! He knows what He's talking about.