Thanks for the question.
To be an instrument of the Church within the context of salvation, means to use ones own free will to freely choose to "be in Christ" and to help and assist others to come to know the Lord and His saving Church.
This was taken from another answer and may help you understand the meaning:
We know when we pray to Mary, as attested to in the Scriptures, she will always lead us to her Son, not herself. (John 2:5) We say Jesus was the effective means of salvation; meaning Jesus packs the punch, but Mary is the instrumental means of salvation.
When you share the Gospel with a non-Christian, you are an instrumental means of salvation for them as well and because you are "In Christ", you are a co-mediator with Jesus called to preach the Gospel to the world. (1 Timothy 2:1-4) Just read the end of Matthew's Gospel where the Apostles were sent. (Matthew 28:16-20) We are called to continue what they started. In the same way, Mary was the ultimate instrumental means of salvation for all mankind because without her, "Yes!", to the angel Gabriel, Jesus could never have become man, according to the Father's Plan.
Nevertheless, neither you nor Mary saved the non-Christian by ourselves, but both of you helped in assisting that non-Christian to know and learn about who Jesus is and the Catholic Church He founded on St. Peter. (Matthew 16:13-20)
Mary is an instrumental means of our salvation, in the same way, the saint's are.
The Gospel could never have lasted over 2,000 years after Our Lady's Assumption, without the "yes" from saints and martyrs of the Church to preserve and perpetuate the Catholic faith to this day.
- Have the prayers and works of evangelization by these saints in the past 2,000 years, undermined the sole mediation of Christ?
<No!> Because they are In Christ, they manifest and make present Jesus Christ from generation to generation, as He commanded us. (Matthew 28:16-20)
