How can a man, who knows that he is in God and has eternal life, not try (out of thankfulness) to do God's will?
Christians do the will of God because they are saved, not to be saved. So I'm not sure I understand your word "need".
I feel that I personlly need to do the will of God for many reasons but not in the same sense as "need" to keep God Law as if I were still under law.
Paul teaches that all things are permissable.
Christains as free from law and sin and guilt.
John writes in 1 John 5:13 "I write these things to you who believe in the name of the Son of God so that you may know that you have eternal life."
Question, how long is "eternal life"?
Peace, Philippians 4:7