Why does the trinity matter? What does it have to do with anything? Every Christian has asked questions like that at some point. And while “everything” would be an appropriate answer, here is a more specific one.
For God has done what the law, weakened by the flesh, could not do. By sending his own Son in the likeness of sinful flesh and for sin, he condemned sin in the flesh, in order that the righteous requirement of the law might be fulfilled in us, who walk not according to the flesh but according to the Spirit. Romans 8:3-4 (ESV)
Father, Son, and Spirit each play a distinct yet inseparable role in our salvation. The Father sent the Son to be a perfect sacrifice for our sins and raised him from the dead to give us he hope of eternal life. The Son, together with the Father, sent the Spirit to seal our salvation and to enable us to walk in newness of life. Only a God who eternally exists as three in one can save in this way.