One of the best examples in the Bible of what God thinks about humans borrowing and adapting false (pagan) practices and customs (for holidays like Christmas) for use toward worshipping Him is in Exodus 32. This example occurs after God uses Moses and Aaron to free the children of Israel from Egyptian slavery.
God calls Moses up the mount to receive instructions and the Ten Commandments engraved in stone. The children of Israel must wait at the foot of the mountain for him to return. As time passes without his appearance, the Israelites grow restless. When Aaron, the High Priest, sees this he tells the people to bring him their jewelry so that he can melt it down and make them an IDOL (shaped like a calf) for them to worship. The people responded immediately when this was done, as did God when he saw it!
' . . . and they (the people) said, "These are your gods, O Israel, who brought you up out of the land of Egypt!" When Aaron saw this, he built an altar before it; and Aaron made proclamation and said, "Tomorrow shall be a festival to the LORD." '
'The LORD said to Moses, "Go down at once! Your people, whom you brought up out of the land of Egypt, have acted perversely . . . Now let me alone, so that my wrath may burn hot against them and I may consume them"' (Exodus 32:4-5, 7, 10, NRSV)
Notice that God did not accept the worship of the Golden Calf as worship directed to Him despite Aaron proclaiming a festival to him (verse 5) would occur the next day. God's anger was kindled toward the Israelites for introducing pagan practices into their worship of Him. God was ready to DESTROY the Israelites, the people He recently freed from Egypt, for such disobedience until Moses intervened (Exodus 32:11-14).
Customs used to worship false gods that are transferred to the true God are unacceptable since the Bible says God never compromises with paganism. This is exactly the point the apostle Paul was making in 1Corinthians 10:19 - 22. What the pagans practiced did not honor the true God, regardless of how much sincerity or faith they had. The same goes for the customs used in holidays like Easter and Christmas.
How did pagan customs and holidays like those involved with Christmas come into the CHRISTIAN church? They primarily entered through the Catholic Church's efforts to adopt then "Christianize" certain pagan holidays NOT found in the Bible so that they could attract more people in their pews on Sunday. The Catholics began to paganize themselves especially after the fourth century A.D. when the Edict of Milan openly endorsed Christianity as a religion. The truth is, the pagan festivals celebrated around the time of the winter Solstice have much more to do with Christmas than the birth of Jesus (which, truth be told, occurred early in the fall).