If God doesn’t change, then why not follow both testaments?

Answer:

"Jesus Christ is the same yesterday, today, and forever. Do not be carried about with various and strange doctrines. For it is good that the heart be established by grace, not with foods which have not profited those who have been occupied with them" (Hebrews 13:8-9).

The point made by the writer of Hebrews is that God is not fickle. He doesn't command one thing one day and another the next. However, you assume something greater than what the Bible states. You assume that God is fixed and unable to do anything different.

Though God retains His character and nature, it does not follow that those whom God must deal with are fixed in moral integrity. Man changes and how God deals with man changes. "Then the LORD saw that the wickedness of man was great in the earth, and that every intent of the thoughts of his heart was only evil continually. And the LORD was sorry that He had made man on the earth, and He was grieved in His heart. So the LORD said, 'I will destroy man whom I have created from the face of the earth, both man and beast, creeping thing and birds of the air, for I am sorry that I have made them.'" (Genesis 6:5-7). Yet, God is still true to His nature and remains the same.

The fact that God remains the same also does not mean that God cannot implement plans to save man in stages. When Jesus was with his disciples he did not tell them everything at once. "I still have many things to say to you, but you cannot bear them now. However, when He, the Spirit of truth, has come, He will guide you into all truth; for He will not speak on His own authority, but whatever He hears He will speak; and He will tell you things to come" (John 16:2-3). Jesus did not change, but it took time for his disciples to change into people Jesus wanted them to be.

When God gave the Law of Moses to Israel, it was another step in His plan to save mankind. It wasn't meant to be a permanent solution to the problem man presented by their sins. In fact, their continued sins under the Old Testament demanded further changes in how God dealt with man. What you are choosing to ignore is that God said the Old Testament was a temporary step toward a permanent solution.

"Behold, the days are coming, says the LORD, when I will make a new covenant with the house of Israel and with the house of Judah- not according to the covenant that I made with their fathers in the day that I took them by the hand to lead them out of the land of Egypt, My covenant which they broke, though I was a husband to them, says the LORD. But this is the covenant that I will make with the house of Israel after those days, says the LORD: I will put My law in their minds, and write it on their hearts; and I will be their God, and they shall be My people. No more shall every man teach his neighbor, and every man his brother, saying, 'Know the LORD,' for they all shall know Me, from the least of them to the greatest of them, says the LORD. For I will forgive their iniquity, and their sin I will remember no more." (Jeremiah 31:31-34).

The writer of Hebrews, which you quoted to "prove" that God could not change His covenants, discusses this passage from the Old Testament at length to prove just the opposite.

"For on the one hand there is an annulling of the former commandment because of its weakness and unprofitableness, for the law made nothing perfect; on the other hand, there is the bringing in of a better hope, through which we draw near to God" (Hebrews 7:18-19).

"But now He has obtained a more excellent ministry, inasmuch as He is also Mediator of a better covenant, which was established on better promises. For if that first covenant had been faultless, then no place would have been sought for a second" (Hebrews 8:7-8).

"In that He says, "A new covenant," He has made the first obsolete. Now what is becoming obsolete and growing old is ready to vanish away" (Hebrews 8:13).

"And for this reason He is the Mediator of the new covenant, by means of death, for the redemption of the transgressions under the first covenant, that those who are called may receive the promise of the eternal inheritance" (Hebrews 9:15).

"Previously saying, "Sacrifice and offering, burnt offerings, and offerings for sin You did not desire, nor had pleasure in them" (which are offered according to the law), then He said, "Behold, I have come to do Your will, O God." He takes away the first that He may establish the second" (Hebrews 10:8-9).

Because they are two different covenants, they cannot both be followed. While there are similarities, the two covenants have different terms and conditions. You cannot follow both any more than a woman can have two husbands.

"Therefore, my brethren, you also have become dead to the law through the body of Christ, that you may be married to another--to Him who was raised from the dead, that we should bear fruit to God" (Romans 7:4).

To try to follow the old only pulls you away from the new.

"Stand fast therefore in the liberty by which Christ has made us free, and do not be entangled again with a yoke of bondage. Indeed I, Paul, say to you that if you become circumcised, Christ will profit you nothing. And I testify again to every man who becomes circumcised that he is a debtor to keep the whole law. You have become estranged from Christ, you who attempt to be justified by law; you have fallen from grace" (Galatians 5:1-4).