score:4
St. Thomas Aquinas commenting on this verse writes:
And fall from the simplicity that is in Christ. That is simple which lacks composition. Therefore, the false apostles formed one sect with Judaism and the Gospel, commanding that the ceremonies of the law be observed along with the Gospel. Therefore, they fall from the simplicity of Christ, who, being seduced by the false apostles, observe those ceremonies along with the Gospel; and this the Apostle feared about the Corinthians: the integrity of the just guides them (Prov 11:3)
Aquinas' Commentary on 2 Corinthians C11 L1
Thus, Aquinas sees "simplicity" as being opposed to a form of duplicity or composition between the Gospel and the Mosaic law. The early church was plagued by a form of heresy known as Judaizing whereby following the law of Moses was presumed required of the baptized.