In this context, Jesus is referring to being rich in carnality. In other words, Jesus had no problem with the "rich man" being financially wealthy. He was addressing the young man in parabolic or metaphorical language, inviting him to reject and surrender his worldly and carnal way of thinking to the "poor," meaning those who are devoid of Divine revelations. The "poor" alludes to Satan, his army, and anyone who is more worldly/carnal than spiritual. Jesus was inviting the "rich man" to relinquish the carnality of the system of this fallen Adamic world and to be baptised in his Holy Spirit--Genesis 1:1, the timeless eternal dimension.
1 Corinthians 2:4-16 My (Apostle Paul) message and my preaching were not with wise and persuasive words, but with a demonstration of
the Spirit's power, so that your faith might not rest on men's
[worldly and carnal] wisdom, but on God's power. We do, however, speak
a message of wisdom among the advanced in Divine understanding, but
not the [carnal] wisdom of this age or of the rulers of this age
(Satan and his army), who are coming to nothing. No, we speak of
God's secret wisdom, a wisdom that has been hidden and that God
destined for our glory before time began. None of the rulers of this
age understood it, for if they had, they would not have crucified the
Lord of glory.
However, as it is written: "No eye has seen, no ear has heard, no mind
has conceived what God has prepared for those who love him"--but God
has revealed it to us by his Spirit. The Spirit searches all things,
even the deep things of God. For who among men knows the thoughts of a
man except the man's spirit within him? In the same way no one knows
the thoughts of God except the Spirit of God.
We have not received the [carnal] spirit of the world but the [Holy,
sinless, most pure] Spirit who is from God, that we may understand
what God has freely given us.
This is what we speak, not in words taught us by [carnal] human wisdom
but in words taught by the [pure] Spirit, expressing spiritual truths
in spiritual words.
The man without the Spirit 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.
The spiritual man makes judgments about all things, but he himself is
not subject to any man's judgment:
"For who has known the mind of the Lord that he may instruct him?" But
we have the mind of Christ.