Here we refers to Christian marriage, a marriage between a Christian man and a Christian woman. It is distinct from a natural marriage, a social institution, with its own civil effects.
Marriage is a human reality, a secular one. In one form or another it existed always. But for the believers in Christ it has a new dimension, as He raised it to a sacrament. It signifies and brings grace, it is a marriage “in the Lord” (I Cor 7:39).
Christian marriage is between the baptised man and woman. However, the fact of baptism alone is not enough. They must really believe in Christ and in the sacramentality of marriage. We cannot simply assume that all baptised are believers in Christ and in His teaching. The parish priest or his delegate must verify it. He should see if the proposed couples are actual believers. He should instruct them about various aspects of Christian marriage and prepare them for Christian marriage.