Does God or can God use a sinner to deliver his message to the church or to his people?
I suppose God could use anyone to deliver his message. Today, though, he delivers his message through the Bible. “God, who at various times and in a variety of manners spoke in time past to the fathers by the prophets, has in these last days spoken unto us by his Son, whom he has appointed heir of all things.” (Hebrews 1:1-2)
If the message brought to us by Jesus and the Holy Spirit is repeated by sinners, it is still the message of God. Two passages come to mind in this regard.
“And John answered and said, Master, we saw one casting out devils in thy name; and we forbad him, because he followeth not with us. And Jesus said unto him, Forbid him not: for he that is not against us is for us.” (Luke 9:49-50)
“Some indeed preach Christ even of envy and strife; and some also of good will: The one preach Christ of contention, not sincerely, supposing to add affliction to my bonds: But the other of love, knowing that I am set for the defence of the gospel. What then? notwithstanding, every way, whether in pretence, or in truth, Christ is preached; and I therein do rejoice, yea, and will rejoice.” (Philippians 1:15-18)
If sinners teach what is in the Bible, it is still true, and God rejoices that his word is spoken. If righteous people teach what is not in the Bible as if it were, then they are false teachers, no matter how righteous their lives. It is not the one who carries the word of God that matters; it is that the word of God is preached.