Nothing can be altered without basing it on something... the degree of truth is strong in the original texts of the Hebrew Bible which was destroyed by Constantine... it was revised to serve his political ambitions... Using the Religion of the followers of Eassauh AS at that time made sense in his pursuits of power.
If you have read the bible in it's entirety and reflected on it... with a clear understanding of the Natural Moral Law... you will be able to see the consistency of the nature of Eassah AS.
One way of knowing if it really was Easauh AS (Jesus Christ) who said it and not the writers of the revised and altered Bible... is to check if it doesn't contradict the Qur'an and Islam. Since the Injil is supposed to confirm the Qur'an - and this is according to the Qur'an itself.
So if a verse in the Bible confirms and supports what is in the Qur'an... then it should be considered some of the truths which constantine and Paul wasn't able to filter out or cannot filter out.
Moreover, it doesn't really matter much anymore if it was really Eassauh AS who said it or not... what matters really is what was said. We have to evaluate, weight, analyse and reflect on the words that was supposedly said. If it doesnt contradict, if it confirms, if it's consistent... then it's right. If it creates confussion... then there's something wrong with it.
We can also say that the present day bibles are scrambled messages of the original text... the right way of reading it is to read between the lines... much the same manner a computer would be reading or descrambling a coded or encrypted message.
Much of the truth and conviction comes after reflecting on what we read and of course after asking for divine guidance.
I think that this has a whole mass of wishful thinking. If something agrees with Islam then I agree it has the potential of being the words of Eesa, but we cannot say they are. Just like if a hadith agrees with the Qur'an then that hadith has the potential to be the word of the Prophet, but other factors play a part for us to be sure, such as the chain of narrators, a factor which is missing in the Biblical writings.