It has often been said that Christians do not believe in any one proposition in order to be saved. They believe rather in a person (the heresy of so-called Christian monotheism suggests belief in God through Jesus). Some imply belief in a narrative; God's narrative of His saving deeds in Israel and Christ.
The sacred scripture of John speaks otherwise. It says: "But these are written that you may believe that Jesus is the Christ, the son of God, and that by believing you may have life in his name" (20:31; cf. 1 Jn.5:1).
To believe that Jesus is the Christ is to believe in a proposition about said person. Propositions are declarative or textually stated facts about an entity. Actually, the saying "Jesus is the Christ" meets the criterion of any definition one may propose for the word proposition. And we are told by the scriptures to set our faith in this fact.