I like the point that is made here about the deaf and dumb man being a symbol for us ... isn't that so often the way? We just don't "get it" until God opens our eyes and our heart.
Sacred Scripture quite often shows the laying on of hands as a gesture indicating the transfer of power or blessing (cf. Gen 48:11; 2 Kings 5:11; Lk 3:13). Everyone knows that saliva can help heal minor cuts. In the language of Revelation fingers symbolized powerful divine action (cf. Ex 8:19; Ps 8:4; Lk 11:20). So Jesus uses signs which suit in some way the effect he wants to achieve, though we can see from the text that the effect -- the instantaneous cure of the deaf and dumb man -- far exceeds the sign used.
In the miracle of the deaf and dumb man we can see a symbol of the way God acts on souls: for us to believe, God must first open our heart so we can listen to his word. Then, like the Apostles, we too can proclaim the magnalia Dei, the mighty works of God (cf. Acts 2:11). In the Church's liturgy (cf. the hymn Veni Creator) the Holy Spirit is compared to the finger of the right hand of God the Father (Digitus paternae dexerae). The Consoler produces in our souls, in the supernatural order, effects comparable to those which Christ produces in the body of the deaf and dumb man.