Theophylact of Ohrid

The apostle said "who looks into," and not "who enters into," for the spiritual law, possessing everywhere preeminence and grandeur, knows how to attract even with the briefest reading of it. To the words "the perfect law" he added "of liberty" to point out its distinguishing characteristic — freedom, for the law of Christ, having freed us from fleshly bondage, establishes the one who comes to Him in liberty, makes him more attentive through this freedom, and frees him from the forgetfulness that is harmful to everything good.

Commentary on James ↗

Oecumenius

But he that has looked in the law of perfect liberty, and has remained in it, not made a forgetful hearer, but a doer of the work: this man shall be blessed in his deed. "But he that has looked in." Consider what it says: Who has looked in, and not, Who entered. …

Commentary on James ↗

Bede

But he who has looked into the perfect law of liberty and has continued in it. By the perfect law of liberty, he means the grace of the Gospel. For the law brought nothing to perfection (Hebr. VII). And elsewhere: "For you have not received the spirit of bondage again to fear, but you have received the Spirit of adoption of sons" (Rom. VIII). And again: "Where the Spirit of the Lord is, there is liberty" (II Cor. III). And the Lord Himself: "If the Son therefore shall make you free, you shall be free indeed" (John VIII).

Commentary on the Catholic Epistles ↗