But the natural man does not receive the things of the Spirit of God

1 Corinthians 2:14
But the natural man does not receive the things of the Spirit of God, for they are foolishness to him; nor can he know them, because they are spiritually discerned.

There are things concerning God, including the gospel, that cannot be understood by sheer intellect. God has to open our eyes for us to understand these. God does not hide these things to withhold salvation from people. Yet, God's process is that we should ask Him first, then our eyes will be opened to many truths that are spiritual and real. After salvation, the Holy Spirit keep assisting us in this understanding of more revelations.

And I will give you rest

Matthew 11:28
.... and I will give you rest.

We may be faced with many things from time to time that causes unrest in our body, soul and spirit. If this is allowed to go on undealt with it could damage us physically as well as spiritually. Sometimes we simply can solve the particular problem or sometimes it is us who are stuck at the problem. Only the Lord can give us rest. He may comfort us through the Holy Spirit and / or take us above the problem so we can see things from His point of view instead of ours.