The commandments, “You shall not commit adultery,” “You shall not murder,” “You shall not steal,” “You shall not covet,” and whatever other command there may be, are summed up in this one command: “Love your neighbor as yourself.” Love does no harm to a neighbor.
-Romans 13:9-10a, NIV
Cheaters have been known to profess their commitment to Christianity while continuing in committing adultery.
This passage from Romans makes it plain that the two are incompatible. He leaves no room for ambiguity.
You cannot continue committing adultery and really be a follower of Christ. Jesus calls us to love our neighbor (see Mark 12:31) and committing adultery is the exact opposite of such.
People can make all sorts of claims. Making a claim does not make it so, though.
Look to their actions!
A person truly committed to following Christ is concerned as to whether or not they are loving their neighbor.
This means they are concerned when told their actions are harming their spouse who most definitely is their neighbor. They do not continue the harmful behavior if they are truly committed to following Christ.
It really is that simple.
A cheater may claim to be committed to following Christ while their actions say otherwise. Believe their actions!