I remember learning about your "Sunday Best." Your "Sunday Best" were the best clothes you wore to church on Sundays. The closest Bible verse I've found to support wearing your "Sunday Best" is 1 Timothy 2:9a, "Also the women are to dress themselves in modest clothing, with decency and good sense."
Jeans and t-shirts could be considered, "modest clothing." Here are three trends I've noticed with how people dress at church.
1). People don't frown upon casual clothes anymore at church. Back in the good ol' days if a man wore jeans to church, he might have received a second look or sarcastic comment. Today, people are just glad you came.
2). Our culture has become more casual. My favorite clothing label is Brooks Brothers (the oldest men's clothier in the U.S. while outfitting 39 of the 44 U.S. presidents, including Barack Obama), but I don't know how they stay in business. They were founded in 1818, and their clothes reflect it (see picture above). Now, you only hear about, "Dress for Success" for a job interview. Comfortable clothes is the new norm.
3). Many men don't own a suit. They might have bought one for a formal or wedding years ago - but can't seem justifying spending the money for a suit they'll only wear once or twice. Many men have told me the reason they don't wear a suit and tie to church is because they don't have one (at least one that fits).
The attitude today about dressing up for church is nonchalant - if you want to dress up, then wear your Father's Day tie. If you don't want to dress up, then come as you are.