Where does the Scripture say that? It does not say that.
What if I just provide an example of where Scripture commands us to find joy or pleasure in God? "Delight yourself in the LORD..." (Ps 37:4). That passage can be summarized to say "Desire God, and he will give you himself (that which you desire)." Or to say it in more eloquent terms:
Psa 73:25 Whom have I in heaven but you?
And there is nothing on earth that I desire besides you.
Psa 73:26 My flesh and my heart may fail, but God is the strength of my heart and my portion forever.
The problem is that "enjoying him forever" may be true in the eternal state when we are sinless in spirit and body, however, in the present state that statement is too vague and can be interpreted to mean various things.
So the problem is that it is open to interpretation??? I think Piper made himself clear when he wrote a book about it! So it is not open to interpretation. That is a ploy.
Your issue w/ sin now and sinlessness in the future doesn't hold theological water. Because we have been made alive in Jesus and raised and seated w/ Jesus and given the Holy Spirit as a downpayment of future grace... we can certainly glorify God in our mortal bodies much in the same way as in resurrection. Otherwise, why do we have the commands to enjoy God? Why did the Psalmist speak as if God was his ultimate treasure?
In the present state, enjoying God over everything else is exactly the hallmark of sanctification. Allow C. S. Lewis to illustrate: "It would seem that Our Lord finds our desires not too strong, but too weak. We are half-hearted creatures, fooling about with drink and sex and ambition when infinite joy is offered us, like an ignorant child who wants to go on making mud pies in a slum because he cannot imagine what is meant by the offer of a holiday at the sea. We are far too easily pleased.” By finding ultimate pleasure in God over against sex or drugs or fill in the blank... we glorify God.
However, the scripture is not vague that glorifying God means to attribute only to Him which honors him - "I always do that which pleases the Father" and Christ is the model for glorifying God is he not? Is not the reverse equally as true that sin is coming "short of the glory of God"?? Hence, God is most glorified when His will is being accomplished in our life and that is the true meaning of Biblical joy and enjoyment of God.
When I enjoy God, what am I doing if not attributing to him that which honors him. I witness b/c I desire God. I sing b/c I find pleasure in God. I preach b/c I find ultimate satisfaction in God. We are creatures of desire. I am desiring the greatest thing in the universe. As a result, my actions best display his glory... b/c I serve him out of desire rather than duty.