Quote
The efficacy of baptism is not tied to that moment of time wherein it is administered; yet, notwithstanding, by the right use of this ordinancy the grace promised is not only offered, but really exhibited and conferred by the Holy Ghost, to such (whether of age or infants) as that grace belongeth unto, according to the counsel of God's own will, in his appointed time.

Westminster Confession of Faith, Baptism

Do modern Calvinists still believe this? If so, how can they have any fellowship with those who deny that baptism is a efficacious means of grace?

BTW, the WCF article is in agreement with the Lutheran Book of Concord.