One of the most influential evangelical Christians in the US has called for full inclusion of gay couples into the life of the Church.

Baptist minister Tony Campolo, who was once a spiritual advisor to former president Bill Clinton, has always been a progressive evangelical but in spite of that has until now spoken out against gay marriage.

It is time to include gay couples fully into the Church, he says. The "exclusion and disapproval" they receive from the Christian community must now end, Campolo insists. He warns that the Church is in danger of repeating the same kinds of mistakes it made when it supported slavery and opposed the ministry of women.

Just 18 months ago Campolo said: "My position has been, over the last several years, that the government should not legitimate gay marriage, and it should not legitimate heterosexual marriage."

But now, in a post on his own website, he has reversed this position and is urging full acceptance.

Raised as a strict evangelical, Tony Campolo's strength throughout his ministry has been his deep knowledge and understanding of Scripture.

Calling for the Church to be more welcoming to gay people, he says: "As a young man I surrendered my life to Jesus and trusted in Him for my salvation, and I have been a staunch evangelical ever since."

He says he places his highest priority on the words of Jesus, emphasising Matthew 25 where Jesus makes clear that on Judgment Day the defining question will be how each of us responded to those he calls "the least of these".

Because of Campolo's known concern for social justice and for the oppressed, he says he has often been asked in recent years if he is ready to accept fully into the Church those gay Christian couples who have made a lifetime commitment to one another.

He admits he has until now remained ambiguous. "One reason for that ambiguity was that I felt I could do more good for my gay and lesbian brothers and sisters by serving as a bridge person, encouraging the rest of the Church to reach out in love and truly get to know them. The other reason was that, like so many other Christians, I was deeply uncertain about what was right," he confesses.

He explains it has taken hours of prayer, study, conversation and emotional turmoil to get to the point where he is ready to change his mind and finally call for the full acceptance of Christian gay couples into the Church.

He admits that for some Christians, the sole purpose of marriage is procreation, which obviously negates the legitimacy of same-sex unions.

"Others of us, however, recognise a more spiritual dimension of marriage, which is of supreme importance. We believe that God intends married partners to help actualise in each other the 'fruits of the spirit', which are love, joy, peace, patience, kindness, goodness, faithfulness, gentleness and self-control, often citing the Apostle Paul's comparison of marriage to Christ's sanctifying relationship with the Church."

This doesn't mean that unmarried people cannot achieve the highest levels of spiritual actualisation – Jesus himself was single, after all – but only that the institution of marriage should always be primarily about spiritual growth, Campolo argues.

Through his wife Peggy he says he has come to know many gay Christian couples whose relationships work in much the same way as their own marriage.

"Our friendships with these couples have helped me understand how important it is for the exclusion and disapproval of their unions by the Christian community to end. We in the Church should actively support such families. Furthermore, we should be doing all we can to reach, comfort and include all those precious children of God who have been wrongly led to believe that they are mistakes or just not good enough for God, simply because they are not straight."

At 80 years old, he can remember when the Church made a strong biblical cases for keeping women out of teaching roles in the Church, and when divorced and remarried people were also often excluded from fellowship altogether on the basis of Scripture. "Not long before that, some Christians even made biblical cases supporting slavery."

Many of those people were sincere believers, but most Christians now agree that they were wrong. "I am afraid we are making the same kind of mistake again, which is why I am speaking out," he concludes.