"Why do the gospels contradict each other in some instances?"
Firstly, we should acknowledge that Christians would have been very unlikely to purposely change their scriptures. It’s their sacred text! The scandals are when people try to make two authors agree when they clearly don’t. The problem is in that method of interpretation: literalism.
Each gospel writer was writing for a different audience. Matthew and Luke aren’t going to say the same things because they’re talking to two sets of people. Matthew’s writing for Jews. Luke is writing for Hellenistic Greeks.
Why does Luke say ‘The Sermon on the Mount’ takes place on a plain when Matthew says it takes place on a mountain? Because blah blah blah these texts are untrustworthy and full of BS? No.
Because blah blah blah maybe Jesus was on a mountain and then progressed down to a plain... or he was on a plain in front of a mountain... theycouldbothbetrue! No.
In Matthew, every important sermon is on a mountain. Mountains are used in Matthew because he’s writing to a Jewish audience and he’s making comparisons with Moses. It gives Jesus authority. Luke puts the sermon on a plain because he’s talking to Greeks who will want Jesus looking at them level (equality!).
If you give the Bible its context, it makes a lot more sense. Liiiiiiike “Be perfect, therefore, as your heavenly father is perfect.” The Greek doesn’t mean perfect, it means living to your maximum potential. Perfection makes sense to Jews, who give the Law their all. It doesn’t make sense to Greeks whose ethics are more situational. Sooooo Luke changes it to "compassionate," because that is the ethical apex in Greek thought. From the very beginning, by the way, we are seeing Christianity and it’s practice and conceptualization being interpreted differently and through various cultural lenses, not in monotonous Latin.
Again, at Jesus’ empty tomb, the gospels “disagree” about how many angels were there. 1)The variance shows you that the Biblical authors weren’t trying to keep strictly historical or literal accounts, 2) it shows you that folks weren’t trying to “fix” the texts by making everything match up, and 3) it is again variance because of the audience. In Judaism, only one angel delivers a message. It’s how it’s always been and they ain’t changin’ now. Again, Luke, writing to a Greek audience, has two angels because two witnesses mean more in Greek/Roman law. When we give the Bible its context, it means a lot more and makes more sense.
And when we stop deciding, based on our own notions of historicity, to discredit a text that was written in a different time, in a different place, and for a different people, and decide that perhaps we should learn more about that time, place, and people, we will find out that the Bible has more to say than we thought it did, has different things to say than we thought it did, and that MANY churches are wrong about what they’re telling us. Once we realize this, we are free to discover what a God-filled life can be: love.