There is much misunderstanding in the secular world of the Bible and its doctrine. One main misconception surrounds the Old Testament and the New Testament, and how the two are compatible. Consider this question posed by a friend of mine:
Doesn't the fact that there is an OT and an NT, and that the NT is the "new covenant with God" make God fallible? It seems to scream "I messed up, here are the new rules."
Not at all.
You must understand the historical context of God's covenant with Abraham and Israel before making the assertion that the "new" covenant was made because there was something wrong with the old one. That is the fallacy of joint effect (or post hoc ergo prompter hoc). Essentially, one thing is held to cause another when in fact both are the effect of a single underlying cause.
The question implies that the timeline overrides the underlying cause: that because there was a "new" testament, therefore the "old" was outdated and necessitated a new. The old caused the new to be warranted.
This is not the case within the biblical context of the OT and NT. The Old Testament did not cause the New Testament in the same way that our judicial system uses shepherding of legal precedent to "update" judicial law for the current age. God was not "updating" the law through the New Testament, but rather, fulfilling it.
Look at the historical context (as stated in the Bible itself):
Beginning with sin entering the world through Adam and Eve (Genesis), mankind was enslaved to sin with no hope of ever being righteous within himself to merit entering heaven and being with Christ. This is the doctrine of Total Depravity.
The law of the Old Testament was established by God to empirically demonstrate how mankind cannot merit righteousness. The law was not put forth as a legalistic mandate, but to exemplify beyond doubt how far mankind falls short of the standard of God and His righteousness, thereby proving to us our need for a savior.
Because the savior had not yet come, the laws were given to show the bondage of attempting to merit salvation. It was impossible. (This is where the timeline is necessary, but not so far as to outweigh the underlying cause – sin.) The Bible speaks of Abraham's bosom, a "holding place" for those in OT times who believed that the savior would one day come. They couldn't merit salvation by keeping the OT law, but if they had faith that the one would come to fulfill the law and pay the price for all of their sins, then they would be "held" in Abraham's bosom until the savior came.
The point of juxtaposition is Christ. He came to fulfill the Old Testament, and upon His coming, the one sacrifice was indeed made for all of mankind, and we can now, in the New Testament, enter directly into the presence of the Lord upon earthly death.
"To be absent from the body [is] to be present with the Lord." ~II Corinthians 5:8
The New Testament is now post-Christ's earthly sacrifice, and thus we no longer have to attempt to keep all the laws as an external realization of our utter failure. The price of our righteousness has been filled, and the law filled. Thus, the New Testament is in place, a "new" covenant.
As Jesus said, it is "a new covenant in my blood." And Hebrews makes several references to Jesus as the mediator of the new covenant through His blood-His sacrifice.
This new covenant in Christ is not a "fix" to the Old Testament law, but rather to pay the price which the law established was necessary. We now live in "the age of grace" as you may have heard Christians refer to. What this means is that we now are saved through faith that Christ came (past-tense) as savior, whereas before, it was faith that He would come (future-tense).
The main point is to realize the fallacy of joint effect. The "cause" has always been sin, and the Old and New Testaments are not different "attempts" by God to fix this problem. The cause, the problem, the standard and the ultimate fulfillment have always remained the same.
Now that the Savior has come and fulfilled the price demanded by the law (demanded by God's righteousness), we have an advocate in Christ and the wrath of God is no longer upon us because of His sacrifice.