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In his own work Concerning Rebaptism (1528), Luther thrashes the Anabaptists. They had over-emphasized the subjective and downgraded the objective side of baptism. Yet, Luther retorted, important as faith is — it is the Word, and not faith, which is the basis of baptism. Any would-be baptizer who regards faith on the part of the baptizee as essential for the validity of the baptism — can never consistently administer baptism. For he can never be certain that faith in Christ really is present.
It is possible, concedes Luther, that some might conceivably doubt the validity of their own infant baptisms. For they might well have no irrebuttable evidence that they even then already truly believed. They might then conceivably wish to request (re-)baptism — when adults.