Baptism is not a symbol; it's the actual way we contact the blood of Christ.
Baptism isn't "a public declaration of faith." The Ethiopian eunuch was baptised in the desert with just him and Phillip to witness it. The Philipian jailor and his household were baptised in their house with just them, Paul and Silas.
If you have fallen away from the faith since being baptised, there is no more grace for you. If you have merely backslidden, that is, are living a sinful life but still believe in Jesus Christ, then I urgently tell you to forsake those sins and return to serving God fully. It is not necissary for you to get rebaptised, provided you were baptised in the right manner and for the right reasons in the first place.
If you were baptised in the wrong manner (sprinkling, pouring) or for the wrong reasons (without turning from your sins first, as just "a public declaration of faith", "an outward sign of an inward grace", then you definitely need to be re-emersed, for the remission of sins, understanding what I have said about baptism in this post and the post below.