Soli goes with Mariae, so you can translate soli Mariae veraci as "(in) truthful Mary alone". Sōlus is one of the adjectives with an unusual declension where the genitive singular ends in -īus and the dative in -ī for all genders. There is a mnemonic "UNUS NAUTA".

Soli goes with Mariae, so you can translate soli Mariae veraci as "(in) truthful Mary alone" (as brianpck points out, "alone" in this case needs to be understood as an adjective rather than an adverb, so it would have been clearer for me to offer a translation like "Mary on her own/by herself"). Sōlus is one of the adjectives with an unusual declension where the genitive singular ends in -īus and the dative in -ī for all genders. There is a mnemonic "UNUS NAUTA".