People bash Christianity by talking about the Old Testament. And it is true that Christianity is essentially a New Testament religion that is based primarily on the teachings of Christ and his apostles. However, the Apostles used the Old Testament to teach about Lord Jesus Christ, not the New Testament. The New Testament was in the process of being written by the apostles as they were teaching out of the Old Testament. Consider the points that follow.
Firstly, most atheists do not know what they are talking about. They think meaningless lives are significant, but only in the eyes of self-opinionated fools. What meaning is there in being born just to die like a rotting fly-blown carcass? Many atheists think dolphins are intelligent, yet they are found fly-blown, rotting and dead on beaches around the world. Atheists are no different. They have no hope.
Secondly, the Old Testament declares that God has a Son. The Son of God in the New Testament is the Son of God in the Old Testament.
Thirdly, the Old Testament has God saying, "I the Lord, do not change."
Fourthly, the God of the Old Testament says that there is no other God besides Him. Admittedly, people make gods out of many things: wood, stone, hobbies, sports, celebrities, heroes, occupations, wealth, status, ideals, and beliefs, etc. But these are not Divine. For they are not the Creator God, Lord of the Universe. They are merely mirages or temporal images.
Fifthly, Israel was supposed to be a theocracy that would demonstrate righteous living to the other the nations, unfortunately for them, they rebelled and rejected the Lord God of Heaven and Earth in preference for a human ruler. (In some respects,Muslims give a minor indication of how a theocracy may have functioned; only Allah is not the Name of God, and the god they serve is the god of this world.)
Sixthly, the Old Testament is still relevant today, for there are unfulfilled prophecies that are waiting to be fulfilled, and the New Testament refers to the Old Testament as the writings with which the faith of Christians needs to conform. Consequently, those who attempt to interpret the Bible without recourse to the Old Testament are in error.
Seventhly, the Only Words written by the God of the Universe are the Ten Commandments and these have not been done away with although many people wrongly think so and quote Matthew 5:17 but overlook:
For truly, I say to you, till heaven and earth pass away, not an iota, not a dot, will pass from the law until all is accomplished. (Matthew 5:18 RSV)
Eighthly, the Apostle to the Gentiles pointed out that the Law was unable to bring about God's requirements, therefore:
God has done what the law, weakened by the flesh, could not do: sending his own Son in the likeness of sinful flesh and for sin, he condemned sin in the flesh, in order that the just requirement of the law might be fulfilled in us, who walk not according to the flesh but according to the Spirit. (Romans 8:3-4 RSV)
Ninthly, the flesh is a reference to human desires that we all possess and those who live by them cannot please God, because they constantly inflict pain upon other human beings for one reason or the other. References to ungodly living are made in the Old Testament and references are also made in the New Testament. Consequently we have this dichotomy between the lifestyle of the flesh and the spiritual lifestyle that brings peace of mind. While the just requirements of the Law are fulfilled in those who walk by the Spirit of God, this is not the case for those who walk according to the dictates of the flesh; the Law of God being the benchmark. This is why we read
For those who live according to the flesh set their minds on the things of the flesh, but those who live according to the Spirit set their minds on the things of the Spirit. To set the mind on the flesh is death, but to set the mind on the Spirit is life and peace. For the mind that is set on the flesh is hostile to God; it does not submit to God’s law, indeed it cannot; and those who are in the flesh cannot please God. (Romans 8:5-8)
Tenthly, the Law of God, as expressed by the Ten Commandments, not only tells us what is acceptable to God but also what is required of us if we are to obtain salvation. More importantly, since these are the only words written by God Himself, anyone attempting to interpret the Bible without regard to these will find themselves in error. These commandments were written by God's Own Finger and that Finger points at you, whoever you are. If you do not live up to them, then whatever you say is similar to what gets flushed down the sewer. "Live up to them, and then I will listen to you. Until that time, you are hostile to me," is basically what the Lord God says. Most people calling themselves Christians, do not walk in the Spirit of God and therefore are easily mocked by scoffers because they do not know where they themselves stand before the Lord of Life.