But read 1 John. That book's raison d'etre is expressed this way:
(1:4) ... these things write we unto you, that your joy may be full ... (5:13) These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God.John mentions a lot of things that help give us confidence, the primary one being that we have faith in Christ. John never mentions baptism in this epistle.
Leithart should learn from 1 John that Baptism is not what provides us our assurance.