Colossians 1:13 describes physical saints as already having been "translated" into
the Kingdom. As such, this passage seems to imply that Christians are now
in the Kingdom of God. However, this clearly isn't the case, since 1 Corinthians 15:50 tells us that "flesh and blood [physical bodies] cannot inherit the
kingdom of God."
Part of the confusion here comes from the meaning of the word kingdom.
In addition to meaning a literal kingdom, the Greek word basileia, translated "kingdom," denotes
sovereignty and royal power (Vine's Complete Expository
Dictionary of Old and New Testament Words, "Kingdom").
This passage in Colossians shows that God's sovereignty and power begin
in the life of the Christian at conversion. The New International Version
Study Bible explains that in this verse the word kingdom "does not here
refer to a territory but to the authority, rule or sovereign power of a
king. Here it means that the Christian is no longer under the dominion
of evil (darkness) but under the benevolent rule of God's Son."
Virtually all other occurrences of basileia, when referring to the Kingdom
of God, point to the literal dominion that Christ will establish at His
return (Matthew 6:33; Revelation 11:15). As "heirs of God" in training
to inherit that future Kingdom (Romans 8:15-17; Matthew 25:34; Revelation 20:4,6), Christians are thus already subject to the sovereignty and authority
of that Kingdom, although not yet residents of it.