The book The Cross of Christ by Anglican priest John Stott really details the earliest forms of worship by Christians and it included crosses and the sign of the cross. Also, he goes through the NT and it's various uses of the word cross and finds MANY instances of it being used in praise of God and Christ. https://www.amazon.com/Cross-Christ-John-Stott/dp/083083320X/ref=sr_1_2?crid=N69E8L1AUC4U&keywords=the+cross+of+christ+by+john+stott&qid=1569817335&s=gateway&sprefix=the+crooss+of+chris%2Caps%2C237&sr=8-2