“What is the opposite of antinomianism?”
Would it be fair to assume that the instinctive response … would be “Legalism”? It might be the right answer at the level of common usage, but it would be unsatisfactory from the standpoint of theology, for antinomianism and legalism are not so much antithetical to each other as they are both antithetical to grace. This is why the scripture never prescribes one as the antidote for the other. Rather grace, God’s grace in Christ in our union with Christ, is the antidote to both.
The wholesale removal of the law seems to provide a refuge [for the antinomian]. But the problem is not with the law, but with the heart – and this remains unchanged. Thinking that his perspective is now the antithesis of legalism, the antinomian has written an inappropriate spiritual prescription. His sickness is not fully cured. Indeed the root cause of his disease has been masked rather than exposed and cured. There is only one genuine cure for legalism. It is the same medicine the gospel prescribes for antinomianism: understanding and tasting union with Jesus Christ himself. This leads to a new love for and obedience to the law of God, which he now mediates to us in the gospel. This alone breaks the bonds of both legalism (the law is no longer divorced from the person of Christ) and antinomianism (we are not divorced from the law, which now comes to us from the hand of Christ and in the empowerment of the Spirit, who writes it in our hearts).
From The Whole Christ: Legalism, Antinomianism, and Gospel Assurance – Why the Marrow Controversy Still Matters by Sinclair B. Ferguson, pg. 156, 157