This is a fantastic point. The problem is, the truth is one, God is one but the faith through which you see this truth is not the truth itself. If you acknowledge the existence of God, that truth is not relative. Relativism creeps in when you confuse the faith for the truth.
You can read parts of the Bible and easily come to different conclusions. I can read the Gospel of Mark and say that the inspired writer didn’t think that Jesus was divine. When I read John, I have to conclude that the writer wanted us to believe that Jesus is God. Arius was adept at using scripture to argue his points. Arianism was a widely held view at one time. It took a Church council to “lay down the law” on what constituted the proper view of the “truth.”
It is like the three blind men and the elephant. One has the tail, the other the nose and another say the ear. Each concludes differently what type of animal the elephant really is. The elephant is still an elephant. To say that we perceive things differently is not to acknowledge relativism.
God is God, I find nothing inconsistent with God’s omnipotence or omnibenevolence to say that he sent different messengers (who translated the messages) to different peoples. If fact, given the beauty and massive diversity of creation, I would expect it. Anything less might imply a lack of creativity, effort or compassion.