Tim Keller addresses the common objection to Christianity, that of the inconsistency of Christians following only the rules from the Bible which seem to suit them. For example, the Old Testament sacrificial laws and laws about stoning disobedient children are ignored but the laws about prohibiting homosexual behaviour are not.
Keller explains that the essential morality in the Old Testament continues in the New Testament and in the teachings of Jesus, however the ceremonial laws and laws about ritual cleanliness are no longer in action as they have been fulfilled in Jesus.
He also explains that the punishments for moral crimes in the Old Testament are different to those for today. This is because of the difference between the religiously controlled state law of Old Testament Israel and the church instituted by Christ which is a part of all nations and cultures under their own state rule.
Keller finishes with the observation that, “If Christ is God, then this way of reading the Bible makes sense,” and that those objecting to Christianity on the grounds of their misunderstanding of what seems to them to be inconsistency; those people are asking Christians to deny this core of Christianity in order to make Christianity not make sense, i.e. they make an assertion without logic behind it.
Christianity, properly understood, must lead to picking only some rules to follow from the Old Testament.
The full article is required reading for a deeper understanding.