When we speak of it as being the "instrument" we are speaking in causal terms.
There are different types of causes:
formal: the design or idea
material: the material means
instrumental: the instrument
efficient: chief agent causing it
sufficient: equal to the task of causing it
final: purpose
Think of a sculpture. The formal cause is the mental or drawn image that the sculpture is made according to. The material cause is obviously the material (marble, ice, etc.) that the sculpture is made from. The instrumental cause is the instrument (chissel, chainsaw, etc) the sculpture is made with. The efficient cause could be seen as the sculptor. The final cause is the reason the sculptor sculpted. The sufficient cause could also be seen as the scupltor.
There is a sense in which all of these things caused the sculpture.
In the case of salvation, the instrumental cause of our salvation is faith. The material cause of our salvation is Christ's righteousness. The Holy Spirit is both the efficient cause (in applying the work of Christ to us) and the sufficient cause (in regenerating our hearts and quickening us to faith). The Covenant of Redemption between the persons of the Trinity is the formal cause, and God's glory is the final cause.
There is a sense in which all of these things can be said to cause our salvation.
In looking again at what you said, I think what you may have heard actually was that God uses
His Word as the instrument to "bring" men to salvation. Faith is the instrument God uses to
actually save men (justifying them via the expiation of their sins and the imputation of Christ's righteousness to them).
Anyway, hopefully that has answered your question. If you have further questions, please feel free to post them in the "Ask a Calvinist" subforum and we'll be more than happy to answer them to the best of our ability