In the following Mounce citation (Mounce Greek Dictionary, Version 3.0), Mt 3:11 should be Mt 13:11
GK G3696 | S G3466 μυστήριον mystērion 28x
a matter to the knowledge of which initiation is necessary; a secret which would remain such but for revelation, Mt. 3:11; Rom. 11:25; Col. 1:26; a concealed power or principle, 2 Thess. 2:7; a hidden meaning of a symbol, Rev. 1:20; 17:7 → mystery; secret.
Matt. 3:11 “I baptize you with water for repentance, but he who is coming after me is mightier than I, whose sandals I am not worthy to carry. He will baptize you with the Holy Spirit and fire.
Matt. 13:11 And he answered them, “To you it has been given to know the secrets of the kingdom of heaven, but to them it has not been given."