One obedience? He obeyed more than one time...many different commands...
the text states one man's obedience not a man's (christ's) one obedience...
And yes many will be made righteous...many unspecified...as He didn't die for one specific group...but all who have sinned...so all man kind...
Those that believe He died and rose and for them are a group which forms after the event...
Those before the event believed it would be secured by One...
So since He obeyed are we not to also do the same?
All that Jesus did and all that Jesus was, was for our salvation and justification. We are complete in him, Colossians 2:10, No religion needed.