The writer of Hebrews tells is "Without Faith it is impossible to please God!" (Heb 11;6) It seems to me that Faith and Belief work together. We are told "Believe in the Lord Jesus Christ and you will be saved." Believing assumes that there is Faith present. So Faith is essential for Salvation (does that make sense?). Faith is essential for our transition from unbelief to becoming a believer and disciple of Jesus Christ - by which we receive all the benefits of sonship, the presence and power of the Holy Spirit and Heaven to come.
How does sin trick us? I believe that it is not SIN that tricks us. It is our enemy SATAN who is behind this manoevre to weaken our faith and witness for Christ. We are at war - in a spiritual battle with the unseen forces of evil. He tricks us by whispering in our mind, "Go on once will not harm you." or, things like "no-one's watching, go ahead." We must always stand strong - and remember that our Heavenly Father is watching over us and it does matter.
How does it harden us? Satan gets us to do it once - then He's got us hooked. The next time we agree with his logic and do it again it weakens our faith and we lose our desire to read His word (the Bible) ... not sure why - but its true in my experience. We lose the desire to pray and the net effect is to become hardened, a little at a time, towards the things of God.
What is the value of Christian fellowship to keep our faith strong? Christian fellowship is everything to us to stand strong. The precious fellowship, 'Koinonia' of the saints is emphasized many times in the Bible. Take a hot coal out of the fire and leave it alone for awhile and it soon loses its glow. Its the same for us. We all need the encouragement of other believers to stand strong in the battle.
God's rich Blessings on you all - Denis