1 Peter 1:14-16 never says that Gentiles are to keep the Mosaic covenant, certainly never mentions kashrut. Nor could it since Peter himself argued at the council of Jerusalem not to burden Gentile converts with these laws.
Can you be more specific? Where in Lev 23 does it mention Gentiles, and if it says they belong to God rather than Israel, wouldn't that make my case rather than yours?
I didn't say anything to advocate that Gentiles keep the Mosaic Covenant, far from it. In 1 Peter 1:16, it doesn't say that Gentiles are to have a holy conduct because that is what the Mosaic Covenant requires or because they should behave like Jews, but rather it says that Gentiles are to have a holy conduct because God is holy. We are not under the Mosaic Covenant, but we are still under the same God, so unless you are advocating that God's holiness changed between covenants, then the way to have a holy conduct in line with His holiness remained same, which involves keeping kashrut. In 1 Peter 1:16, it references Leviticus 11:44-45, which is talking about kashrut, and is part of God's instructions for how to have a holy conduct, which Gentiles are told to do.
The Mosaic law is God's instructions for how to do what is holy, righteous, and good (Romans 7:12) and the followers of God are to do what is holy, righteous, and good not because we are under the Mosaic Covenant, but because that is how to act in line with the holiness, righteousness, and goodness of the God that we serve. It Leviticus 23:2, it says that they are God's appointed feasts, and while the chapter doesn't mention Gentiles, Gentiles are still to keep them not because of any covenant or because of who the Jews are, but because of who God is. It should be fairly straightforward that keeping God's holy days is part of what it means to have a holy conduct.
It's not advocating Gentiles keep the Mosaic Covenant. It's simply dealing with hypocrisy. Paul was another who argued at the council of Jerusalem that Gentiles not be burdened with Mosaic law.
Do you think it is important for Gentiles to have circumcised hearts? If so, then obedience to the Mosaic law is the way to demonstrate that, which is again only advocating obedience to the law, not to the covenant. Anyone who wants to do what is holy, righteous, and good can do so by obeying the Mosaic law even if they aren't under any covenant agreement with God to have such a conduct, but the New Covenant is indeed one where we are instructed to have such a conduct (1 Peter 1:14-16, 1 John 3:10, Ephesians 2:10).
According to Deuteronomy 4:2, it is a sin to add or subtract from God's law, so if Paul had tried to subtract all but four of the laws for Gentiles, then he would have been sinning. Furthermore, according to Deuteronomy 13:4-6 the way to tell whether someone is a false messenger of God is if they teach God's people against obeying what He had commanded, so if you think that Paul did that, then you should consider him to be a false prophet and disregard what he said. We must obey God rather than man, so if Paul had argued not to burden Gentiles with obedience to God, then we should obey God instead of him. However, Paul said that the Mosaic law was given to reveal to us what sin is (Romans 7:7) and he was clear that Gentiles are not permitted to do what God has revealed to be sin (Romans 6:15), so he was in favor of Gentiles obeying it. Paul also said that our faith does not abolish God's law, but rather it upholds it (Romans 3:31), so he never argued against keeping it, though he did argue against requiring Gentiles to keep Jewish traditions in order to become saved (Acts 15:1). The Mosaic law does not require Jews to keep or their traditions for how to keep it in order to become saved, so that was never the reason why we should obey it in the first place. It does not follow that because we shouldn't obey God's law for a reason for which it was never intended that therefore we shouldn't obey it for the many reasons for why it was given.
