How does a New Testament Christian determine what Old Testament sins are also relevant in the New Testament?
It’s clear that the New Testament has replaced the Old (Jer. 31:31-34; Heb. 8:7-13; Rom. 7:1ff; Heb. 9:15; et al). When one studies both the Law of Moses in the Old Testament and the laws of Christ found in the New Testament, it won’t take long before one finds striking similarities…and differences.
For example, all of the Ten Commandments given to Moses on Sinai are basically repeated as commandments given to Christians in the New Testament with the exception of the Sabbath Day commandment (Ex. 20:1-17; cf. Matt. 22:37; 1 John 5:21; Matt. 6:9; Eph. 6:2; 1 John 3:15; Heb. 13:4; Eph. 4:28; Col. 3:9; 3:5). Concerning the Sabbath commandment, as well as the similar commandments concerning the other holy days and seasons as well as the dietary restrictions and similar directives of the Law of Moses, Christians were specifically commanded to not teach that observing them was God’s command under the New Covenant (Col. 3:16-17), that such things were to be in place “until the time of reformation” (Heb. 9:10) which took place when the Law of Moses was taken out of the way at the cross (Col. 2:14; Eph. 2:14-16). However, the New Testament placed no such similar limitations on the commandments which had to do with morality and ethics, such as the laws against stealing, anger, idolatry, strife, drunkenness, murder, adultery, fornication in general (including homosexuality), lying, and the like (cf. Gal. 5:19-21; 1 Cor. 6:9-11).
Interestingly, a study of Genesis and the first 19 chapters of Exodus (all of which covers the time in human history before the Law of Moses was put into effect for Israel at Mount Sinai) shows that these moral and ethical directives found in both the Law of Moses and the laws of Christ were also in place before Sinai. For example, homosexuality and adultery were both considered sinful (Gen. 19:4-7; 39:7-10), as was murder (Gen. 4:8-16).
Therefore, a good rule of thumb to follow when it comes to determining what is considered sinful in both the Old and New Testaments is to see if the action under question is condemned as unlawful not only under the Law of Moses, but also under the Law of Christ. Check to see if it was also considered unlawful during the time before the Law of Moses also. That should help you determine if an Old Testament sin is still a sin under the New Testament.