Nothing in the old testament serves as law for Christians today (Gal 3:24-25, Heb 7:12) – not principle, precept, ordinance, commandment, statute – nothing. The “new testament” is held out as our complete law for today (II Cor 3:6, Heb 9:15, Heb 7:22). But it is still so very important that we study the old testament regularly because there are vital things we can learn from it (I Cor 10:11) that help us understand the new testament. Some examples include …
John 3:14-15 compares Jesus being hung on the cross and the necessity of our belief in him to the Num 21:4-9 story of the brazen serpent being hung up and the necessity of the Israelites to look at that serpent in order to be healed of their snake bites. If we never study the old testament, how would we properly understand this parallel? And there are many other cases like that – where the new testament draws a parallel to the old.
I Tim 2:9-10 tells us to dress modestly and Gen 3:7,10,21 helps define for us what modest dress is.
Gen 1:1 tells us that God created the heaven and the earth. The old testament is littered with many such facts that (by definition) don’t change – since they are facts, not laws.
The new testament tells us homosexuality is sinful (Rom 1:26-27), but Lev 18:22-23 in the old testament informs us that God considers the sin like (in the same category as) having sex with an animal.
Ezek 3:18 helps us understand that we should warn those in sin as we have opportunity – though what is and what is not defined as sin has changed in some particulars since Ezek 3:18 was written.
We can’t learn how to worship God today from Lev 10:1-2 since it is part of old testament law, but we certainly can learn from the passage that God will be displeased with us if we don’t worship him the way he has instructed in his new testament law.
There are probably dozens and dozens of other illustrations, but the above should suffice to make the point. The old testament was written “for our learning” (Rom 15:4). Let’s not neglect it in our studies.