The first written form of Finnish was created by Mikael Agricola, a Finnish bishop in the 16th century. He based his orthography on Swedish, German, and Latin. Later the written form was revised by many people.
The Reformation marked the real beginning of writing in Finnish. In the 16th century major literary achievements were composed in Finnish by people like Paavali Juusten, Erik Sorolainen, and Jaakko Finno, as well as Agricola himself. In the 17th century books were written in Finland in Finnish, Danish, Norwegian, Estonian, Latvian, German, and Swedish. However, the most important books were still written in Latin. Finnish and Swedish were small languages of lesser importance.
The basis for the numerous conventions in the Finnish standard language is found in Agricola's work, particularly with respect to spelling. Agricola's language was based on Western Finnish, thus that phonology found its way into the standard Finnish spelling.
Agricola used dh or d to represent the voiced dental fricative [ð] (English th in this) and tz or z to represent the unvoiced dental fricative [þ] (the th in thin). Later, when these sounds disappeared or changed in the different dialects, no one knew how to pronounce them. (Today, the [ð] sound is only in a few particular accents in Western Finland.) However, the spelling remained unchanged, so the standard language pronunciation of d and z was loaned from German (z = /ts/ and d = /d/), producing the "soft D" problem (see Finnish phonology). Later, z came to be written ts. In the standard language, [ð] remained [d], e.g. sydän. In the eastern part of Finland, [ð] became j, v, or disappeared. In the west, it became r, l or d. The sound [þ] became ht or tt (e.g. meþþä --> mehtä, mettä) in the east and some Western dialects, but became ts in the standard language and many Western dialects (meþþä -->metsä).
Agricola made up some words during translation of the New Testament. Some of these words are still in use, e.g. armo "mercy", vanhurskas "righteous". Agricola used about 8500 words and 60% of them are still in use.
Either ch, c or h were used for the voiceless velar fricative (the ach-laut, /x/). In modern Finnish, the difference between /x/ and /h/ has been lost in phonemic terms; while velar friction might appear in 'h', spelling does not reflect it. For example, Agricola's spelling techtin becomes modern tehtiin. Agricola used gh or g to represent the voiced velar fricative. This sound was later lost and also suppressed in spelling, except if it appeared intervocalically, when it became 'v'.