It's nothing new. I know an Episcopal "minister" who married a Hindu and are raising their kids in both "faiths". It's part of the "I'm okay, you're okay, we're all okay" movement much like that coexist bumper sticker.
Look at the old testament -- the Israelites were commanded to destroy the inhabiters of Israel when God gave it to them, but they left stragglers. Those stragglers kept their Ashora poles (sp?) and continued to worship their false God's leading to Israel's judgement before God again and again throughout the book of Judges.
"Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind. This is the first and great commandment." (Matthew 22:37-38)