I have recieved many mixed messages from Christians regarding the OT. One on hand, many argue that the dietary laws, practises such as circumcision and obeying the Sabbath do not need to be observed.
Old Testament, Old Law. The law is fulfilled by Christ. The covenant of Christ and its commandments are in the NT. Ultimately, the new law is written on the hearts and minds of believers (this is said in both OT and NT). Comparing NT to OT can be confusing at times because despite using the same terms, they can mean vastly different things (e.g. "neighbour", "Israel", "commandments").
There are the first two commandments: "love God" and "love your neighbour". And there are forms of the Mosaic 10 commandments that exist in the NT (with some differences). Some dietary rules do exist in the NT, e.g. though nothing in itself is unclean if someone esteems something to be unclean, it is unclean to them (
Romans 14:14).
In the new covenant, one need only look to the NT commandments, but you are free to observe OT rules.
"Circumcision is nothing, and uncircumcision is nothing, but the keeping of the commandments of God." - 1 Corinthians 7:19 KJV. As an extension of "honouring father and mother" there are also implications that other traditions are permitted as well, so long as they don't contradict the law of Christ:
"Where there is neither Greek nor Jew, circumcision nor uncircumcision, Barbarian, Scythian, bond nor free: but Christ is all, and in all." - Colossians 3:11 KJV
Some denominations carry forward old traditions from converts of other religions. Some have a high reverence for OT practices that are no longer required such as circumcision, some have patron saints that were previously identified as gods but now are recognised as servants of God (e.g. the Celtic goddess Bridget was replaced by Saint Bridget in Roman Catholicism to aid Irish conversion and allowed converts to keep aspects of their traditions).
Paul talks about this in the NT, "an idol is nothing" (
1 Corinthians 8:4) in respect to graven images and the like. But if one were to weaken someone's faith by being in the presence of what appears to be idolatry, a believer is to abstain for the sake of the brother or sister that would have their conscience tempted.
Many other Christians argue homosexuality should not be practised on the basis that it is forbidden in the OT.
"Surely you must know that people who practice evil cannot possess God’s kingdom realm. Stop being deceived![a] People who continue to engage in sexual immorality, idolatry, adultery, sexual perversion,[b] homosexuality, fraud, greed, drunkenness, verbal abuse,[c] or extortion—these will not inherit God’s kingdom realm." - 1 Corinthians 6:9-10 TPT
Instead of "homosexuality" (as it is a relatively new term) other translations have "the effeminate" or "pederasty". You don't need the OT to find these passages.