Were Peter and John Illiterate?

Now when they saw the boldness of Peter and John, and perceived that they were uneducated and untrained men, they marveled. And they realized that they had been with Jesus. (Acts 4:13)

The phrase "uneducated and untrained" can be also rephrased as "not educated in rabbinic law nor professionally trained in matters of religion." But, certainly, it does not mean that Peter and John were illiterate. They were literate enough to write epistles and professionally trained fishermen. However, the scribes and temple leaders didn't recognize them as literate according to their standards and procedures of education. But, they were trained better than the scribes in understanding the law and the prophets and their fulfillment in Christ.

Later on, Peter stresses the importance of a right training for an understanding of the scriptures. He says that there are some who are unlearned and unstable that twist the scriptures.

There are some things in those [epistles of Paul] that are difficult to understand, which the ignorant and unstable twist {and} misconstrue to their own utter destruction, just as [they distort and misinterpret] the rest of the Scriptures. (2Pet. 3:16, Amp)

It is more important to know the grace of Jesus Christ than to memorize 10,000 commentaries on the law. The disciples knew Jesus, and everybody took note that they were with Him.