In the Middle Ages when many people were unable to read or write, documents were often signed using the X.

Kissing the X represented an oath to fulfil obligations in the document.

The X and the kiss eventually became synonymous - hence the X at the end of a letter to signify kisses (which we all use)

http://www.stpauls-kst.com/our-worship-customs
The Cross is the unique symbol of Christians. From ancient times the Sign of the Cross has been used for blessing and for Christians to identify themselves to others. Crosses were used as signatures on documents by both literate and illiterate people.

A scribe would write the names of all signatories to a document, but each would sign with his or her own Cross.

Within Church circles, some people still will put a Cross at the top of a letter or note and will put a Cross after their signature. This is most consistently done by priests, nuns, and brothers. Bishops usually sign with a Cross before their names.

So the Sign of the Cross can be a seal and an agreement.

http://www.eastriding.gov.uk/hyperlinks/pdf/LocalFamilyHistoryEmailVersion.pdf

If both bride and groom sign with a cross, this does not necessarily mean that neither of them could
read or write: a literate bride often signed with just a cross so as not to embarrass her new husband.

http://en.wikipedia.org/wiki/Kiss
Throughout history, a kiss has been a ritual, formal, symbolic or social gesture indicating devotion, respect or greeting. It appears as a ritual or symbol of religious devotion. For example, in the case of kissing a temple floor, or a religious book or icon. Besides devotion, a kiss has also indicated subordination or, nowadays, respect.

In modern times the practice continues, as in the case of a bride and groom kissing at the conclusion of a wedding ceremony or national leaders kissing each other in greeting, and in many other situations.

A potentially intereting side note: although some people "marked", I think in some cases the minister simply wrote the names down and then asked them to make their mark when they were capable of signing. I say that because there are instances where a known individual signed one or more records, but marked on other, subsequent records.

Two observations on this:

1. Whatever its history, the X symbol has become so trivialised in the present day that many younger people put it at the end of a text message, facebook post, etc. although they can sign their names, have no religious belief, and hardly know the person the other end. X is being reduced to superficial slobber.

2. If you do enough family history research into marriages to give you a statistically viable sample, you may well find that the use of X provides evidence of gradual increase in literacy. I have discovered that in my small family sample of 52 (from Glos but outside the Forest in this instance) all 20 parties to marriages between 1850 and 1899 signed their names. In the batch between 1825 and 1849, 12 signed and 6 marked. Between 1800 and 1824, 4 signed and 10 marked. A wider survey of this trend might be interesting in terms of both literacy and demography.