If God is one, why do various religions so often disagree?
The Supreme Person sends many incarnations and prophets to earth. Depending on the audience, different prophets and incarnations of God give more or less knowledge.
This is similar to how a math teacher gives different knowledge according to the grade level of the students. Beginning math has just numerals and symbols but advanced math, like algebra, employs variables represented by letters. Someone unfamiliar with algebra may protest, "What are these letters doing here? This isn't math!" But what they're seeing is only a higher level of math than what they're used to.
For example, Buddha teaches how material desire is a cause of suffering. Krishna also teaches this in Bhagavad-gita. But Buddha did not teach about God, the soul, and their relationship because His audience was not as advanced. Buddha is an incarnation of Krishna, He knows everything, but He only spoke partial knowledge in order to reach His particular audience.
We so often see disagreement between religions due to a lack of understanding of this principle. God is one, but He is infinite, and can be seen from unlimited angles of vision. One thing all religions have in common is the idea of developing love for God as the highest goal of life. Religious people who are focused on this goal have less trouble accepting the validity of others' religious beliefs. Religious people who are focused on lesser goals are more likely to find fault with the beliefs of others, since their understanding of the essence of religion isn't as developed. This is known as the inability to see the forest for the trees.