Believe all of what? Paul’s answer to the jailer when asked “what must I do to be saved” was “believe on the Lord Jesus Christ and you will be saved”. Rather succinct and to the point. My question to you was “what did Paul leave off of his answer”?
OK, I’ll go along with this. What’s optional then? The virgin birth? The divinity of Christ? The Trinity? The miracles? The crucifixion? The Resurrection?, etc, etc,... that’s all beyond that isolated verse. What are we to believe about Christ, everything except that he established a Church and not a library?