Counseling centers say that the first step to recovery from drug or alcohol addiction is admitting that you have a problem. Believe on the Lord Jesus Christ, and thou shalt be saved. Put very simply, it means forsaking any other hope of salvation and putting your trust completely in Him. Believing on the Lord Jesus Christ requires first that you admit your need of Him to save you. Believing on the Lord Jesus Christ also requires that you place your trust completely in Him.
Belief involves more than just agreement.
For example, suppose I tell you that I can swim from New York to Canada right across the top of Niagara Falls, and you might agree that I'm a strong enough swimmer to do so. Suppose then that I tell you that I can perform this feat with you on my back.