The problem of faith is never God's but ours. Sometimes our faith will be weak, and if we have sinned very badly, we may not have the assurance of salvation that we once had. That is why it is important to not continue in a sinful lifestyle. He is greater than our sin and His faith is stronger than ours, but we can never expect to be comfortable and live in sin. David cryed out to God to restore the joy of his salvation when he had committed terrible sins for a king to commit.
If a person is truly elect, they cannot lose their salvation. Jesus said in John 6:37:, "37 All that the Father giveth me shall come to me; and him that cometh to me I will in no wise cast out.
38 For I came down from heaven, not to do mine own will, but the will of him that sent me.
39 And this is the Fathers will which hath sent me, that of all which he hath given me I should lose nothing, but should raise it up again at the last day.
As Henrich said, though, there are many who are deceived about themselves and their salvation. They think it an easy thing to just say they believe and have no actions supporting their confessions. These are never truly saved, but only God knows the heart and can judge who they are. We are commanded to love even our enemies and believe the best for them. This doesn't mean we are blind to their fruit and their sins, if blatant. We are supposed to correct one another in humbleness and love. It doesn't happen that often, though, but we are held accountable even for that if we do not warn a brother or sister about a sin and try to restore them to the path of truth. I guess that I'd say that a person cannot lose what they never had, and that God never loses anything that belongs to Him.
2 Corinthians 13:5 Examine yourselves, whether ye be in the faith; prove your own selves. Know ye not your own selves, how that Jesus Christ is in you, except ye be reprobates?
1 John 3:18 My little children, let us not love in word, neither in tongue; but in deed and in truth.
19 And hereby we know that we are of the truth, and shall assure our hearts before him.
20 ¶ For if our heart condemn us, God is greater than our heart, and knoweth all things.
Psalms 51:10 Create in me a clean heart, O God; and renew a right spirit within me.
11 Cast me not away from thy presence; and take not thy holy spirit from me.
12 Restore unto me the joy of thy salvation; and uphold me with thy free spirit.