Luke 11:52

Woe unto you, lawyers! Ye have usurped, as S. Ambrose renders the Greek ήζατε, the key of knowledge, i.e., the teaching of the law and the interpretation of scripture. Ye have used this knowledge for your own evil purposes, and have prejudiced the people against Me and the salvation which I came to bestow. Thus ye shut up the kingdom of heaven against men: for ye neither go in yourselves, neither suffer ye them that are entering to go in, S. Matthew 23:13.

Thus S. Ambrose and Tertullian; and S. Cyril, who understands the key of knowledge to mean the law, the sign of the justice of Christ, and adds, Faith also is the key, because by means of it we retain the knowledge and the truth, for "unless ye believe ye will not understand." These men therefore shut up the kingdom of heaven, for they neither explained the law as testifying to Christ, nor did they suffer men to believe on Him.

Figurativly, S. Augustine (lib. ii. Quæst. Evang.), alluding to Isaiah 22:22, and Revelation 3:7, says, The key of knowledge is humility, which these lawyers themselves understood not, and were unwilling that others should understand.