1 Corinthians 6:17 says, "But he who is joined to the Lord becomes one spirit with him."
It will be somewhat of a mystery until we see the Lord face to face. When we are born again, or born of the Spirit - both terms Jesus used - the Bible says that our body becomes the temple of the Holy Spirit, who comes to live in us. (1 Cor. 6:19)
We as humans are three part beings. 1 Thessalonians 5:23 says, "Now may the God of peace himself sanctify you completely, and may your whole spirit and soul and body be kept blameless at the coming of our Lord Jesus Christ." The way I understand it is that we are a spirit (our life essence), we have a soul (our mind, will, and emotions), and we live in a body.
Our spirit resides in our body, and when the Holy Spirit comes into us at the new birth, He joins with or co-mingles our own spirit. That is why we are called in 2 Corinthians 5:17 a "new creation." We look like other human beings, but within us dwells the very presence of God.
That means that God is NEVER far from us! He is not somewhere distant in heaven. His line is never busy. He is with us at the very core of our being.
When the Holy Spirit moves in, there is often (but not always) a heightened awareness of God's presence and His love which at times can be almost euphoric - especially in the beginning. The Holy Spirit does not "take over" however. God created us with a free will, and it appears from Scripture that He does not intend to override that will. The Spirit does however, begin to communicate to us the mind and will of God. We begin to be uncomfortable with actions that days earlier we had no problem with. We begin to be aware of the needs of others. We begin to feel "led" in certain God honoring directions. All because of this "oneness" we now have with God's Spirit at the deepest level of our being.
Not only does God not force His will on us, it actually takes some doing on our part to be more and more sensitive to the moving and working of the Holy Spirit within us. As we take time to focus on Him; worship Him, seek Him in prayer; fill our hearts with His word; and as we respond to His leading not hardening ourselves to Him, we become more and more sensitive to His "still small voice."
This "oneness" and intimacy with God is what human beings were created for from the start. Let us determine to take full advantage of this incredible opportunity that Christ has restored to us through the cross.