He asked if Catholics believed that all Christians were priests. I said that we all have a partial sharing of Christ’s priesthood. He then asked why I, as a believing Catholic, can’t perform the transubstantiation of the host and wine into the body/blood of Jesus.
Give him some Scripture:
For as in one body we have many members, and
all the members do not have the same function, so we, though many, are one body in Christ, and individually members one of another. Having gifts that differ according to the grace given to us, let us use them: if prophecy, in proportion to our faith; if service, in our serving; he who teaches, in his teaching; he who exhorts, in his exhortation; he who contributes, in liberality; he who gives aid, with zeal; he who does acts of mercy, with cheerfulness. (Romans 12:4-8)
Now there are varieties of gifts, but the same Spirit; and there are varieties of service, but the same Lord; and there are varieties of working, but it is the same God who inspires them all in every one. To each is given the manifestation of the Spirit for the common good. … For just as the body is one and has many members, and all the members of the body, though many, are one body, so it is with Christ. For by one Spirit we were all baptized into one body – Jews or Greeks, slaves or free – and all were made to drink of one Spirit. For the body does not consist of one member but of many. If the foot should say, “Because I am not a hand, I do not belong to the body,” that would not make it any less a part of the body. And if the ear should say, “Because I am not an eye, I do not belong to the body,” that would not make it any less a part of the body.
If the whole body were an eye, where would be the hearing? If the whole body were an ear, where would be the sense of smell? But as it is, God arranged the organs in the body, each one of them, as he chose. If all were a single organ, where would the body be? As it is, there are many parts, yet one body. The eye cannot say to the hand, “I have no need of you,” nor again the head to the feet, “I have no need of you.” … Now you are the body of Christ and individually members of it. And God has appointed in the church first apostles, second prophets, third teachers, then workers of miracles, then healers, helpers, administrators, speakers in various kinds of tongues.
Are all apostles? Are all prophets? Are all teachers? Do all work miracles? Do all possess gifts of healing? Do all speak with tongues? Do all interpret? (1 Corinthians 12:4-30)