For Lutherans, ask them where specifically in Scripture that we are told that only the clergy can celebrate the Lord’s Supper. I’ve asked this question of a Presbyterian minister. They can’t answer it because it is not there. Of course, only a validly ordained clergy can celebrate the Eucharist (Lord’s Supper to Lutherans) but this was part of the Oral Tradition that the Apostles handed down.
Also, ask them where we are told that there are only two sacraments and not seven.
Regarding infant baptism, it is implicit in Scripture. A cursory reading of Scripture would seem to support the idea that only adults who make a profession of faith in Christ can be baptized. But the early Church believed in baptizing infants and this is where the practice comes from. Yes, I believe there is Scriptural support but it is like the doctrine of the Trinity; it’s not all in one place…you have to dig for it.
The order of worship/liturgy: Lutherans have a liturgy that almost matches the Mass but where did they get it from? (From Catholics, of course.)
Ask them if their ministers/bishops have valid ordination…do they have Apostolic Succession?
That should be enough to plant a seed.