The Bible gives many examples of how "adultery" is committed. The most obvious way is being involved with someone that is not your spouse. But the bible also states that if you lust upon another you have committed adultery in your heart. God commands us to get out of the adulterous relationship, at which time we can be forgiven if we have truly repented. But the one who was adulterous in the relationship is not free to marry again. What if you were free to marry, married someone who was not free to marry (while married you would be in an adulterous relationship? Right?), but then got divorced due to that same person being adulterous again. Are you free to remarry?
Based upon the teaching of Scripture, you are correct in what you say about adultery and remarriage. However, when Jesus allowed remarriage for the innocent spouse when the cause for the divorce is fornication, he was speaking of actual fornication. The Lord’s purpose in Matthew 5:25-28 was not to give a new definition of adultery, but to emphasize that the righteousness of the Kingdom must exceed the righteousness of the scribes and Pharisees. You are correct that a person who married a divorced person who was divorced because he or she was guilty of adultery would also be an adulterer. Your question, as I understand it, is whether if such a person obtained a second divorce also based upon adultery, would a remarriage be in keeping with God’s will. Someone once described a question as being one that would cross a Rabbi’s eyes. I am inclined to put this question in that category. For one things, it requires too many assumptions to properly answer. For instance, did the innocent party from the first divorce enter into the second marriage with full knowledge that he or she would be living in adultery? If so, is the sorrow upon the second divorce Godly sorrow that leads to repentance, or just regret that the adulterous second mate was unfaithful in the innocent party’s second marriage? Frankly, and being as honest as I can, such an inquiry sounds more like someone seeking to justify what they have already made up their mind to do. I cannot see how a person truly interested in the welfare of his or her soul would make such an inquiry. It seems more likely that a person truly seeking to do God’s will and save his or her soul, would not undertake such an uncertain voyage on a sea known to be stormy.
You Must Hear the Gospel
You must hear the gospel and then understand and recognize that you are lost without Jesus Christ no matter who you are and no matter what your background is. The Bible tells us that “all have sinned, and come short of the glory of God.” (Romans 3:23) Before you can be saved, you must understand that you are lost and that the only way to be saved is by obedience to the gospel of Jesus Christ. (2 Thessalonians 1:8) Jesus said, “I am the way, the truth, and the life: no man cometh unto the Father, but by me.” (John 14:6) “Neither is there salvation in any other: for there is none other name under heaven given among men, whereby we must be saved.” (Acts 4:12) "So then faith cometh by hearing, and hearing by the word of God." (Romans 10:17)
You Must Believe
You must believe and have faith in God because “without faith it is impossible to please him: for he that cometh to God must believe that he is, and that he is a rewarder of them that diligently seek him.” (Hebrews 11:6) But neither belief alone nor faith alone is sufficient to save. (James 2:19; James 2:24; Matthew 7:21)
You Must Repent
You must repent of your sins. (Acts 3:19) But repentance alone is not enough. The so-called “Sinner’s Prayer” that you hear so much about today from denominational preachers does not appear anywhere in the Bible. Indeed, nowhere in the Bible was anyone ever told to pray the “Sinner’s Prayer” to be saved. By contrast, there are numerous examples showing that prayer alone does not save. Saul, for example, prayed following his meeting with Jesus on the road to Damascus (Acts 9:11), but Saul was still in his sins when Ananias met him three days later (Acts 22:16). Cornelius prayed to God always, and yet there was something else he needed to do to be saved (Acts 10:2, 6, 33, 48). If prayer alone did not save Saul or Cornelius, prayer alone will not save you. You must obey the gospel. (2 Thess. 1:8)
You Must Confess
You must confess that Jesus Christ is the Son of God. (Romans 10:9-10) Note that you do NOT need to make Jesus “Lord of your life.” Why? Because Jesus is already Lord of your life whether or not you have obeyed his gospel. Indeed, we obey him, not to make him Lord, but because he already is Lord. (Acts 2:36) Also, no one in the Bible was ever told to just “accept Jesus as your personal savior.” We must confess that Jesus is the Son of God, but, as with faith and repentance, confession alone does not save. (Matthew 7:21)
You Must Be Baptized
Having believed, repented, and confessed that Jesus is the Son of God, you must be baptized for the remission of your sins. (Acts 2:38) It is at this point (and not before) that your sins are forgiven. (Acts 22:16) It is impossible to proclaim the gospel of Jesus Christ without teaching the absolute necessity of baptism for salvation. (Acts 8:35-36; Romans 6:3-4; 1 Peter 3:21) Anyone who responds to the question in Acts 2:37 with an answer that contradicts Acts 2:38 is NOT proclaiming the gospel of Jesus Christ!
You Must Be Faithful Unto Death
Once you are saved, God adds you to his church and writes your name in the Book of Life. (Acts 2:47; Philippians 4:3) To continue in God’s grace, you must continue to serve God faithfully until death. Unless they remain faithful, those who are in God’s grace will fall from grace, and those whose names are in the Book of Life will have their names blotted out of that book. (Revelation 2:10; Revelation 3:5; Galatians 5:4)