He did for he said: "For truly I say to you, until heaven and earth shall pass away, not even one iota, ... away, one jot or one tittle will by no means pass from the law till all is fulfilled" Matt 5:18
and he also said
"Therefore anyone who sets aside one of the least of these commands and teaches others accordingly will be called least in the kingdom of heaven" Matt 5:19
Check out 1 Cor 6:9-11 and 1
Thessalonians 4:3. The first command on Gentile Christians Acts 15:20 included "abstinence from sexual immorality."
In short the Jewish moral law applies in toto to all Christians of any cultural background.
BTW the decline in Christianity is due to the hypocrisy of its "self-professing adherents." No-one likes a hypocrite but many would-be "Christians" are just that. One of the problems with Christianity being so widespread is that there is no final authority on earth able to call out people people for falsely calling themselves "Christians." That's not to say that individual churches don't, but too much of the time churches are more interested in philosophical disagreeements over esoteric things such as the "Trinity" or political matters, rather than elementary human morality, without which there can be no Christianity.
As Christ said "By their fruit ye will know them." Matthew 7:15-20