One of the things that is often said by Baha'is is that God gave guidance to all people. There was always some rules or "laws". Some laws were spiritual and everlasting. Others were social laws for a specific time and specific people and were only meant to be temporary. With that a Baha'i can show that things like "love your neighbor", "Don't lie", "Don't steal" are present and true in all religions and therefore are everlasting spiritual laws. From there they can say all religions are one, are from one God and all teach the same thing. But, obviously, religions don't teach the same thing. And, they teach totally different things about who and what is "God".
One of the Baha'is favorite analogies is to say that religions are like the different grades at school. When society was like a little child, it went to 1st grade. Later, it moved on to the higher grades. Now, it's ready for High School or college with Baha'u'llah. The previous teachers taught, Krishna, Buddha, Moses, Jesus, Mohammad etc., people what was needed to get to the next grade, as if there was a logical and consistent flow from one religion to another. After studying just the very basics of several religions, I don't see it. And, I really feel that the Baha'i Faith is essentially saying that all religions of the past are wrong and don't have the teachings that can unite all people. So, in a nice way, they are saying that your religion was fine, but now it's time to turn to Baha'u'llah and leave your old religion behind.