Three things: the mutual help of husband and wife, children, and the prevention of sexual immorality.

The order is worth noticing, because mutual help comes first. The confession is following Genesis 2, where the reason given for making a companion is that being alone is not good, before any mention of children.

Then the Lord God said, 'It's not good for the human to be alone. I'll make someone to help him and be his partner.'

Genesis 2:18

Confession 25.2

Anyone able to give informed consent - and Christians are to marry in the Lord.

The confession is deliberately broad first and then adds a duty for believers specifically: not to marry those who do not share the faith, or those whose life or teaching is plainly against it. It is stated as an obligation on the believer rather than as a bar on anyone else.

A wife is bound to her husband as long as he's alive. But if he dies, she's free to marry anyone she wants, as long as he belongs to the Lord.

1 Corinthians 7:39

Confession 25.3

Yes. Marriage within the degrees of blood relation, or of relation by marriage, that the word forbids - and the confession says no human law and no consent of the parties can ever make such a marriage lawful.

It is unusually absolute for a document that elsewhere leaves room for circumstances. What it points at is the list in Leviticus 18, and behind that, John the Baptist, who lost his head for telling Herod that it was not lawful for him to have his brother's wife.

John had been telling Herod, 'It's not right for you to marry your brother's wife.'

Mark 6:18

Confession 25.4