The first and simplest answer to this question comes to us from the rubrics. The rubrics for the Mass are the instructions given to us in the Roman Missal and they tell us the structure of the Mass. They tell us when to stand, sit, kneel, respond, etc. They also tell us that we are to make the sign of the Cross on our forehead, lips and heart just prior to the reading of the Gospel.
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But your question is probably more about the “why” we do this. The answer is simple. This gesture of signing ourselves with the Cross is a way of praying, “May the Lord be in my mind, on my lips and in my heart.” It is said prior to the reading of the Gospel as a way of prayerfully inviting Christ to come to us in these ways as His sacred Word is proclaimed. The hearing of the Gospel must change our minds, it must be proclaimed by our lips, and it must affect our heart. By heart we especially mean our will, affections, emotions and passions.
So next time you sign yourself with the Cross before the Gospel make sure that it’s more than just an external gesture, make sure it is also a prayer and an invitation to Christ.