By "them" I assume we mean that in Matt 13:11, "He replied, “The knowledge of the mysteries of the kingdom of heaven has been given to you, but not to them."
Jesus is talking to His disciples and the crowds (V1) by the sea. However, in V10 we now have a restricted audience of just the disciples.
"them" is used by Jesus in contradistinction to the disciples; that is Jesus could have said, "The knowledge of the mysteries of the kingdom of heaven has been given to you [ie, the disciples], but not to them [ie, others who are not disciples.]"
The Bible makes this point several times and in different ways such as:
- 1 Cor 2:14 - The natural man does not accept the things that come from the Spirit of God. For they are foolishness to him, and he cannot understand them, because they are spiritually discerned.
- John 16:13 - But when he, the Spirit of truth, comes, he will guide you into all the truth.
It is little wonder that Paul declared in Rom 8:9, "You, however, are controlled not by the flesh, but by the Spirit, if the Spirit of God lives in you. And if anyone does not have the Spirit of Christ, he does not belong to Christ."
Thus, without the gift of the Holy Spirit, we can understand nothing of the Bible's teaching and nothing about the truth of Jesus. Indeed, Paul also says that we do not even know how or what to pray for without the prompting of the Spirit (Rom 8:26).
We have several instances of people in the NT praying for the gift of the Holy Spirit such as: Acts 8:15, Eph 1:17, 3:16, Phil 1:19, 2 Kings 2:16, etc.
Thus, it appears that Jesus' disciples (in the broadest sense) are those with the gift of the Spirit who only are capable of understanding the spiritual principles of the Kingdom of Heaven.
Ellicott observes (comments on Matt 13:11):
To know the mysteries.—The Greek word, like “parable,” has passed into
modern languages, and has suffered some change of meaning in the
process. Strictly speaking, it does not mean, as we sometimes use
it—when we speak, e.g., of the mystery of the Trinity, a truth which
none can understand—something “awfully obscure” (the definition given
in Johnson’s Dictionary), but one which, kept a secret from others,
has been revealed to the initiated. Interpreted by our Lord’s teaching
up to this time, the mysteries of the kingdom may be referred to the
new birth of water and the Spirit (John 3:5), the judgment to be
exercised hereafter by the Son of Man (John 5:25), the power of the
Son of Man to forgive sins (John 9:6), the new ideas (no other word
will express the fact so well) which He had proclaimed as to the
Sabbath (John 12:8), and fasting, and prayer, and alms (John 6:1-18).
Those ideas had been proved occasions of offence, and therefore, for
the present, the Teacher falls back upon a method of more exoteric
instruction.
Barnes also notes that:
To the disciples it was given to know these truths. This was important
for them, as they were to carry the gospel around the globe. To the
others it was not "then" given. They were too gross, too earthly; they
had too, grovelling conceptions of the Messiah's kingdom to understand
these truths, even if communicated to them. They were not to preach
the gospel, and hence our Saviour was at particular pains to instruct
his apostles in the system which they were to preach. The Pharisees,
and Jews generally, were not prepared to receive the system, and would
not have believed it, and therefore he purposely employed a kind of
teaching which was intended for his apostles only.
