"human effort does not oppose God's grace" = TRUE, when the effort is a result of faith in God's grace. For instance, Paul the apostle repeatedly wrote about how hard he worked as a result of God's grace, that is, knowing that God would undertake for him.
God graciously gives us gifts and empowers us to use them according to His will:
Having then gifts differing according to the grace that is given to
us, let us use them: if prophecy, let us prophesy in proportion to our
1 Cor. 3:10
According to the grace of God which was given to me, as a wise master
builder I have laid the foundation, and another builds on it. But let
each one take heed how he builds on it.
1 Cor. 15:10
But by the grace of God I am what I am, and His grace toward me was
not in vain; but I labored more abundantly than they all, yet not I,
but the grace of God which was with me.
2 Cor. 9:8
And God is able to make all grace abound toward you, that you, always
having all sufficiency in all things, may have an abundance for every
So these verses show that grace (which is freely given apart from our meriting them) and human effort are not exclusive. Christians freely access this grace, this giving of God, when they believe God for it, that is, act in faith.
The problem comes in when we labor to be found worthy of the gifts and helps ("See what I've done, Lord? I'm such a good person!") For instance, the Israelites resisted receiving God's graciously given righteousness because they were going about establishing their own by meritorious works (Romans 9:30-32). So acting apart from faith resists God's grace.