Reading a text in a foreign language with no dictionary at all is a thrilling but demanding endeavor. It is not so easy to figure out unknown words by their context only unless you are quite advanced in the language. Apart from a sharp mind and undivided attention, the key success factors are:

• A low percentage of unknown words. Even if you are a Da Vinci Code genius, you will succeed in deciphering a new word more often when the number of new words is low. For instance if I hide the really rare words in the phrase 'Reading a text in a foreign language with no dictionary at all is a XXXXX but demanding XXXXX. ', you can still figure out the general sense of the phrase. But if I hide more word, 'Reading a text in a foreign language with no dictionary at all is a XXXXX but XXXX XXXXX. ', it becomes more obscure. You will be able to read on, but you will be missing part of the text.
• A strong context helps very much. For instance, if you are reading a technical article about spiral staircases, you can reasonably expect unknown words to fall within a fairly short list. The phrase itself will usually let you know which one it is.
• A good knowledge of word roots and cognates will help you guess the meaning of unknown words. These would not be enough in itself, but with additional clues from the context, a root can be all you need to find out what a word means.

Seasoned polyglots are often very enthusiastic about this way of developping their knowledge of a language. I think the reason is their extensive knowledge of cognates in other languages that help them figure out more easily what words mean.