If someone says that all religions have the same teachings, and same basic principles, and you share that this is not the case; religions contradict each other in ways that are irreconcilable to anyone who is an orthodox adherent to their religion of choice. To be able to back up your assertion, simply have memorized a few doctrinal categories and several of the world’s religions’ positions for each of those categories. Spitting out a few examples of what different religions teach, say, in the categories of who God is, what the source and focus of revelation is, and what lies after this life, should be enough to demonstrate some very stark contrasts in beliefs. Clearly, all religions don’t teach the same views on life, God, and the destiny of mankind, but can you quickly demonstrate it?
The following image is a photo I took of a student’s test that asked this basic question. Check out how he answered the question (click the image to enlarge):