Submit means to be in obedience to.
Resist means to rebel against.
These are opposites. But James links them.
We cannot submit to God without resisting the devil.
Nor can we resist the devil without submitting to God.
It works in reverse, too. If we resist God, we end up submitting to what Satan wants for us.
In verse 6 though, James says God gives grace to the humble, that is to those who submit to Him.
What does Satan give?
Submitting to God. Resisting the devil. It should be easier than it is.