Are all unbelieving Jews under the Law? If they do not believe in Jesus, then how can we witness to Jews to believe He was the final sacrifice “to redeem those who were under the law....” as Galatians 4:5 says?
Paul tells us in Romans:
Rom. 10:4 For Christ is the end of the law for righteousness to everyone who believes.
Rom. 10:5 For Moses writes that the man who practices the righteousness which is based on law shall live by that righteousness.
As Pastor Armstrong explains, the Law condemns a person for as long as they are under law. For a Jew who continues to practice the Law today (albeit in a very limited way), they are bound to the Law's requirements, which Paul says means they are forever under condemnation. But for the Jew who places faith in the work of Christ, that Jew comes out from under the penalty of Law. In this sense, all unbelieving Jews (who practice the Law) are under the Law.
Furthermore, the Law came as a part of a covenant, and that covenant is still in effect between Jesus and Israel, as Jesus Himself said:
Matt. 5:18 “For truly I say to you, until heaven and earth pass away, not the smallest letter or stroke shall pass from the Law until all is accomplished.
The Law (and the covenant that delivered it to Israel) are still in effect until all that it requires is fulfilled. Jesus ultimately fulfills all of it on behalf of those who believe, and the fulfillment of that covenant for Israel doesn't happen until His Second Coming. So in that sense, the Jewish people are still "under" the covenant of Law. When an individual Jew believes in Jesus, that person comes out from under the covenant, but the rest of the (unbelieving) nation remains under that covenant for now.
In regard to Galatians 4, Paul was speaking about those who believe and therefore come out from under the Law (as he says in Romans 10):
Gal. 4:4 But when the fullness of the time came, God sent forth His Son, born of a woman, born under the Law,
Gal. 4:5 so that He might redeem those who were under the Law, that we might receive the adoption as sons.
Notice Paul says that Christ redeems those who "were" under the Law but by faith have now received the adoption as sons. So he's speaking about individuals who believe, not the entire nation of Jews.
Of course, any Jew who attempts to live under the Law is greatly limited in practice, since the priesthood and temple no longer exist. The writer of Hebrews says that the absence of these things in our age was intended as a sign to Israel that their covenant with God has been superseded by something greater:
Heb. 8:13 When He said, “A new covenant,” He has made the first obsolete. But whatever is becoming obsolete and growing old is ready to disappear.
Heb. 9:8 The Holy Spirit is signifying this, that the way into the holy place has not yet been disclosed while the outer tabernacle is still standing,
Heb. 9:9 which is a symbol for the present time. Accordingly both gifts and sacrifices are offered which cannot make the worshiper perfect in conscience,
The destruction of the temple was meant to be a sign to Israel that a new and better way to approach the Lord had been revealed in Jesus, so that for all who believe in Jesus they come out from under the condemnation of the Law.