the geneva catechism 4
362. Do these two Sacraments not serve another end? Yes, they do. They are also signs and marks of our profession. That is to say, by them we declare that we are of the people of God, and make confession of our Christianity.
363. How ought we to judge a man who never wishes to use it? He could not be regarded as a Christian, for in so doing he refuses to confess himself as such, and tacitly, as it were, disavows Jesus Christ.
364. Is it sufficient to receive each once? Baptism is only ordered to be received once, and may not lawfully be repeated. But this is not so with the Supper.
365. What is the reason for that? By Baptism God introduces and receives us into His Church. After He has received us, He signifies by the Supper that He wishes continually to nourish us.
366. To whom does it belong truly to baptize and administer the Supper? To those who are publicly charged to teach in the Church. For the preaching of the Word and the distribution of the Sacraments are things conjoined.
367. Is there any certain proof for this? Yes, indeed. Our Lord specially charged His Apostles to baptize as well as to preach (Matt. 28:19). In regard to the Supper. He ordered all to follow His example. Moreover He performed the office of a minister in order to give it to others.
368. But ought pastors, who are appointed to dispense the Sacraments, to admit without discretion all who present themselves there? In regard to Baptism, as it is administered today only to infants, there is no need for discrimination; but in the Supper the minister ought to take heed not to give it to a man whom he recognizes to be entirely unworthy.
369. Why so? Because it would pollute and dishonour the Sacrament.