"Be on God's side because He is always right" means you should align your morals, actions and decisions with God's principles rather than trying to claim God supports your own personal or political agenda. It is a call for humility, suggesting that ultimate truth and justice rest with God. It encourages you to adjust your own mindset to fit what is universally good, rather than assuming God endorses whatever you already want. God is seen as the ultimate standard of truth, justice and morality. To say He is "always right" means His ways and values are perfect, without the flaws or biases that humans have.