No man can say that Jesus is the Lord, but by the HolySpirit. 1Cor. 12:3.
To call Jesus "Lord" is to confess oneself his servant and to seek his honor alone; to act as his messenger or the bearer of his Word and command. Paul refers here chiefly to the office which represents Christ and bears his Word. Where the office answers these conditions and points to Christ as the Lord, it is truly the message of the Holy Spirit, even though the occupant of the office does not in his own person possess the Spirit; the office itself is essentially of the Holy Spirit. Hypocrisy and invention have no place here. One must proceed in sincerity if he would be certain he is Christ's minister, or apostle, and really handles his Word. Only the inspiration of the Holy Spirit can give one this assurance.
All Christians — each in his own sphere — may equally call Christ "Lord." One may be assured he serves Christ if he can call him "Lord," for only by the Holy Spirit is he enabled to do that. Let him try for a single day — from morning until evening — whether or not he can truly say at all times that he is the servant of God and of Christ in what he does. When delivering a sermon or listening to one, when baptizing a child or bringing one to baptism, when pursuing your daily duties, ask yourself if the act is attended by such faith that you can, without misgiving and not hypocritically, nor mechanically, boast — if necessary, die by your boast — that you serve and please Christ therein. This is calling Christ "Lord." Unquestionably you will often feel your heart doubting and trembling over the matter. Flesh and blood is too weak to obtain this glorious confidence; the Holy Spirit is essential.
I often used to wonder that St. Ambrose was so bold as to call himself a servant of Jesus Christ. I supposed we all ought to be terrified at thoughts of this kind, and that none but the apostles might boast of such honor. But the fact is, we must all say to Christ: Thou art my Lord and I am thy servant; for I believe in thee and aspire to be with thee and all the faithful and to possess thy Word and Sacrament. Otherwise Christ will not acknowledge us.