In his letter to the Colossian Assembly, Paul addressed three types of “Gnostic” heresy that were evidently being taught there:
- Greek Dualism, which cast doubts on Jesus because of claims that he was fully human as well as divine. Plato, whose philosophy was revered in much of the Greek world, held that the spiritual realm was “good”, but anything physical, including any human, was intrinsically evil.
- Pagan Pantheism, which held that all physical objects were inhabited by so-called “elemental sprits” that were either good or evil.
- Judaistic practices being urged on non-Jews in the Church.
It is the last issue, above, that I wish to address here. I believe that it included two main sub-issues:
- Either requiring or suggesting conversion to Judaism was viewed by some as a precursor to possession of the “secret things” of Christianity.
- Urging participation in celebrations of Jewish tradition for possession of that Gnostic “knowledge”.
The first of these sub-issues was dealt with primarily in
Colossians 2:11-14 (ESV)
 In him also you were circumcised with a circumcision made without hands, by putting off the body of the flesh, by the circumcision of Christ,  having been buried with him in baptism, in which you were also raised with him through faith in the powerful working of God, who raised him from the dead.  And you, who were dead in your trespasses and the uncircumcision of your flesh, God made alive together with him, having forgiven us all our trespasses,  by canceling the record of debt that stood against us with its legal demands. This he set aside, nailing it to the cross.
Prior to AD 70, conversion to Judaism was a threefold process requiring circumcision; ritual cleansing (immersion, i.e., baptism, in either open water or in one of the thousands of mikvot, or baptisteries, in Israel and the Diaspora); and an atoning sacrifice in the Temple.
In verse 11, above, Paul explains that circumcision is not required of non-Jewish believers, because our circumcision is a spiritual “circumcision of the heart“, a “stripping away [of the] old nature’s control over the body” (CJB translation).
In verse 12, he identifies baptism as a symbolic death, burial and resurrection along with Messiah.
And in verse 14, he states that our sin-debt is cancelled by the cross. Without digging too deeply into Old and New Testament theology, Jesus’ death cancelled all our sins, past present and future, whereas the Old Testament sin offerings were merely a symbolic atonement, or “covering over“, for specific infractions over specific time frames.
Regarding the Jewish Traditions, refer to
Colossians 2:16-17 (CJB)
 So don’t let anyone pass judgment on you in connection with eating and drinking, or in regard to a Jewish festival or Rosh-Hodesh or Shabbat.  These are a shadow of things that are coming, but the body is of the Messiah.
Rosh-Hodesh (“head of the month”) was the lunar new moon celebration held each month to recognize the coming civil, agricultural and religious cycle. Shabbat, of course, is the Jewish “day of rest”, either the weekly Sabbath or a Sabbath associated with another Jewish festival.
Note that Paul is not condemning these traditional celebrations, which have tremendous prophetic and memorial significance to Christians and Jews alike; rather, he is condemning those who shame others in the church who choose to either celebrate them or to not celebrate them.