Each has his or her personal relationship with God. Each person has God's grace. Each person can encourage others, can pray with others, can pray for others, can be joyful that other individuals have a personal relationship with God, can rejoice that other individuals have been saved. But no individual can actually share God's grace with another individual. Christian denominations or churches are groupings of people who give effective encouragement and support to an individual person's relationship with God.
Individuals join the denomination or church in which they find support.
If that encouragement weakens, the person must move on to a different denomination, because it really does not make a difference what denomination a person belong to as long as that denomination gives support and encouragement, and fits the individual's definition of Christian. Each individual must live out his or her commitment each day, each minute. What this means in specific terms is determined by reading, studying, praying with the Bible. By joining with other Bible-based Christians each individual can both know how to live, and create a society where Bible values are paramount. In all of this other people can support an individuals's relationship with God, but cannot effect that relationship, because people cannot actually share God's grace with each other.