The oldest preserved text in Latin alphabet in any Slavic language are the Freising manuscripts; three manuscripts written a dialect of Common Slavic spoken in Alps or Pannonia, and therefore a close relative or a direct ancestor of modern Slovene, but were found in Freising in Bavaria. Most famous (in Slovenia) is the second of the three manuscripts, which is a preacher's call for the faithful to repent of their sins.
First sentence, transcribed directly:
>Eccę bi detd naſ neze
>greſil tevuekigemube
>ſiti ſtaroſti neprigem
>lióki nikoligeſe pet
>ſali neimugi niſlzna
>telezeimoki nuúvuę
>kigemubeſiti
Modern Slovene translation is roughly:
Če bi ded naš ne grešil, bi mu na veke bilo živeti, starosti ne prejeti, nikoli skrbi imeti, ne solznega telesa, temveč na veke bi mu bilo živeti.
It's hard to do direct comparison because of differences in orthography, but since the manuscripts are dated to some point at the end of 10th century, they're not expected to differ much from Common Slavic yet.
The Celovec/Rateče manuscript was written around 1380, when Slovene already started diverging into regional dialects; it is written in Upper Carniolan dialect with some elements of Carinthian (Rateče is right at the border between the two regions). It includes several prayers; I'll give Pater Noster, transcribed from the manuscript, and the modern Slovene.
>Otſcha naſs kyr ſy w nebeſſich
>poſſwetſchenu body twoye yme
>pridi bogaſtwu twoye
>body wola twoya
>kakor w nebeſſich yno na ſemly.
>Kruch naſs wſedanny day nam dannaſs
>yno odpuſti nam dalge naſſe
>kakor yno my odpuſſchamo naſſen dalnykom
>yno naſs ne wuppellay wedner o yſſkuſb
>le naſs reſſy ob od ſlega.
>Amen.
Oče naš, ki si v nebesih,
posvečeno bodi tvoje ime,
pridi k nam tvoje kraljestvo,
zgodi se tvoja volja
kakor v nebesih tako na zemlji.
Daj nam danes naš vsakdanji kruh
in odpusti nam naše dolge,
kakor tudi mi odpuščamo svojim dolžnikom,
in ne vpelji nas v skušnjavo,
temveč reši nas hudega.
Amen.
The text still differentiates between y, Slavic front yer, and i' It also still includes the vocative case (oča instead of otec'', though in modern Slovene, the old vocative became nominative in this and several other words). Otherwise, the text is already easily readable for Slovene speakers, unlike the Freising manuscript one.
The first translation of the Bible into Slovene was finished in 1578, published 1583, and it set the norm for Slovene until the 19th century. It was done by Jurij Dalmatin, a Lutheran preacher. The language he used was based on earlier work by Primož Trubar, another Lutheran preacher, who published his Catechismus and Abecedarium in 1550, and also translated the New Testament. Trubar and Dalmatin still maintained the difference between y and i in certain words, but it wasn't very consistent anymore, so Slovene apparently lost this distinction around that point. However, they were both from Lower Carniola, so there are also geographic differences between the two examples.
Matthew 15:33
>Natu nega Iogri knemu prauio, Kei mi hozhmo tukai vti puſtzhaui tulikan kruha vſeti, de mi tako mnoshizo ludi naſitimo?
(Trubar, 1555-1577)
>Natu ſo k'njemu djali njegovi Iogri: Kej moremo v'puſzhavi tulikajn kruha vseti, de bi tulikajn folka naſsitili?
(Dalmatin, 1578)
>Inu njegóvi Jógri rekó k' njemu: Od kod tèdaj bómo vsęli v' puſhavi tóliku kruha, de ſe ena takú velika mnóshiza naſiti?
(Japelj, 1784-1802)
>Učenci so mu dejali: »Od kod naj v puščavi vzamemo toliko kruha, da nasitimo táko množico?«
(SSP, 1996)
>And his disciples say unto him, Whence should we have so much bread in the wilderness, as to fill so great a multitude?
KJV
(Japelj apparently attempted to use diacritical marks to differ between open and closed e, so closed e is written ę)
Vocative in Slovene was lost at some point at the end of 17th century, causing several names to split; so now we have pairs of names where one is derived from nominative and the other from vocative; examples being Jurij and its vocative Jure (George), Mark and its vocative Marko.