Yes, one can fall back into sin and commit the works of the flesh. That is why there are constant admonitions in the Scriptures to hold fast to the liberty where Christ has made us free and be not entangled again with the yoke of bondage, to remain and abide in Christ so that we could bear much fruit, to walk as He walked, to continue in the faith, to pray, to grow and stand in the grace of God, to put on the whole armor of God so that we would be able to stand against the wiles of the devil, etc.
However, John wrote to those believers who were walking in the light and were not deceived and said:
“My little children, these things write I unto you, that ye sin not. And if any man sin, we have an advocate with the Father, Jesus Christ the righteous.” (1John 2:1)
Notice that John said “IF” any man sin, not “WHEN” you sin. The modern gospel is a gospel of “when you sin.” There is no expectation of a sinless life in Christ. They think so lightly of sin that they boldly, and some even proudly, say that all are sinners and that we sin every day. “If any man sin,” is speaking of “any” man in general, or to “any” man in relation to the whole body of Christ, and not an expectation that all believers in Christ “will” sin. Notice that in the first part of the verse he uses the words “children” and “ye,” which are both plural, and then he shifts to “any” man, which is singular. “If any man sin,” we can only go back to the Father through the shed blood of Jesus Christ. He is our advocate with the Father, who cleanses us from all sin and from all unrighteousness. The glorious gospel of Jesus Christ is not the one of modern-day Christianity where a constant life of “slipping and falling” is taught and expected. It is a victorious life in Christ.
“Now unto Him that is able to keep you from falling, and to present you faultless before the presence of His glory with exceeding joy.” (Jude 1:24)
“Who gave Himself for us, that He might redeem us from all iniquity, and purify unto Himself a peculiar [God’s own special possession] people, zealous of good works.” (Titus 2:14)
“That He might present it [His church] to Himself a glorious church, not having spot or wrinkle, or any such thing; but that it should be holy and without blemish.” (Ephesians 5:27)
“And you, that were sometime alienated and enemies in your mind by wicked works, yet now hath He reconciled in the body of His flesh through death, to present you holy and unblameable and unreproveable in His sight - If ye continue in the faith grounded and settled, and be not moved away from the hope of the gospel…” (Colossians 1:21-23)
“But now being made free from sin, and become servants to God, ye have your fruit unto holiness, and the end everlasting life.” (Romans 6:22)
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Thomas physically saw and touched Jesus, and believed. None of us have seen him in the flesh; Jesus said "blessed are those who have not seen me but believe anyway." I do not believe that God will condemn people for having doubts, or find it hard to accept Jesus, or understand the theology of salvation, or whatever. The people who, I believe, go to hell are any who have had the Gospel clearly explained to them, who have had a chance to hear about a God who loves them perfectly, unconditionally, eternally, completely, who has reconciled them to himself and will never let them go; and say "no thanks". And what's more, go on saying "no thanks" for the rest of their human lives. If someone completely and deliberately rejects what God has done for them through Jesus and is not interested in eternal life, when they die, God will respect the choice they made and they will not spend eternity with him. If they don't want to be with him, they don't want to be with him. 
