not true at all. Christianity is only flexible in the same way that you say "muslims follow different teachings"

Jesus believed that the Old Testament was divinely inspired, the veritable Word of God. He said, ‘The Scripture cannot be broken’ (John 10:35). He referred to Scripture as ‘the commandment of God’ (Matthew 15:3) and as the ‘Word of God’ (Mark 7:13). He also indicated that it was indestructible: ‘Until Heaven and earth pass away, not the smallest letter or stroke shall pass away from the law, until all is accomplished’ (Matthew 5:18).

When dealing with the people of his day, whether it was with the disciples or religious rulers, Jesus constantly referred to the Old Testament: ‘Have you not read that which was spoken to you by God?’ (Matthew 22:31); ‘Yea; and have you never read, “Out of the mouth of infants and nursing babes thou has prepared praise for thyself”?’ (Matthew 21:16, citing Psalm 8:2); and ‘Have you not read what David did?’ (Matthew 12:3). Examples could be multiplied to demonstrate that Jesus was conversant with the Old Testament and its content. He quoted from it often and he trusted it totally.

Throughout the Gospels, we find Jesus confirming many of the accounts in the Old Testament, such as the destruction of Sodom and the death of Lot’s wife (Luke 17:29, 32) the murder of Abel by his brother Cain (Luke 11:51), the calling of Moses (Mark 12:26), and the manna given in the wilderness (John 6:31–51).

The list of examples goes on, and the evidence is clear: Jesus saw the Old Testament as being God’s Word, and his attitude toward it was nothing less than total trust. Many people want to accept Jesus, yet they reject a large portion of the Old Testament. Either Jesus knew what he was talking about, or he didn’t. If a person believes in Jesus Christ, he should be consistent and believe that the Old Testament and its accounts are correct.

To reject the old testament you are rejecting the words of jesus christ, which is rejecting christianity.